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Poc1A and Poc1B act together in human cells toensure centriole integrity

Magali Venoux1,*, Xavier Tait1,*, Rebecca S. Hames1, Kees R. Straatman1, Hugh R. Woodland2 andAndrew M. Fry1,`

1Department of Biochemistry, University of Leicester, Lancaster Road, Leicester LE1 9HN, UK2School of Life Sciences, University of Warwick, Coventry CV4 7AL, UK

*These authors contributed equally to this work`Author for correspondence (amf5@le.ac.uk)

Accepted 9 September 2012Journal of Cell Science 126, 163–175� 2013. Published by The Company of Biologists Ltddoi: 10.1242/jcs.111203

SummaryProteomic studies in unicellular eukaryotes identified a set of centriolar proteins that included proteome of centriole 1 (Poc1). Functional

studies in these organisms implicated Poc1 in centriole duplication and length control, as well as ciliogenesis. Using isoform-specificantibodies and RNAi depletion, we have examined the function of the two related human proteins, Poc1A and Poc1B. We find thatPoc1A and Poc1B each localize to centrioles and spindle poles, but do so independently and with different dynamics. However, although

loss of one or other Poc1 protein does not obviously disrupt mitosis, depletion of both proteins leads to defects in spindle organizationwith the generation of unequal or monopolar spindles. Our data indicate that, once incorporated, a fraction of Poc1A and Poc1B remainsstably associated with parental centrioles, but that depletion prevents incorporation into nascent centrioles. Nascent centrioles lacking

both Poc1A and Poc1B exhibit loss of integrity and maturation, and fail to undergo duplication. Thus, when Poc1A and Poc1B are co-depleted, new centrosomes capable of maturation cannot assemble and unequal spindles result. Interestingly, Poc1B, but not Poc1A, isphosphorylated in mitosis, and depletion of Poc1B alone was sufficient to perturb cell proliferation. Hence, Poc1A and Poc1B playredundant, but essential, roles in generation of stable centrioles, but Poc1B may have additional independent functions during cell cycle

progression.

Key words: Poc1, Cell cycle, Centriole, Centrosome, Mitosis, Spindle

IntroductionCentrioles are remarkably conserved structures found in most

animal cells (Azimzadeh and Marshall, 2010; Kobayashi and

Dynlacht, 2011). In post-mitotic differentiated cells, they take the

form of basal bodies that subtend the axonemal microtubules

found within cilia and flagella, while in dividing cells they form

the core of the centrosome, the primary site of microtubule

nucleation both in interphase and mitosis. They therefore play

essential functions in many aspects of cell biology and

multicellular organisms cannot develop properly in their

absence (Bettencourt-Dias et al., 2011; Gerdes et al., 2009;

Marshall, 2008; Nigg and Raff, 2009).

Centrioles are strikingly beautiful structures, being composed

of nine, highly stabilized, microtubule triplets organized as

blades into a barrel that is ,500 nm in length and 200 nm in

diameter (Azimzadeh and Marshall, 2010). In G1, animal cells

contain two centrioles, an older one called the mother and a

younger one called the daughter. These undergo one round of

duplication per cell cycle to maintain centriole numbers in

dividing cells. Centrioles have a polarity with a proximal end

from which new centrioles grow, and a distal end, which, in the

case of the mother centriole, has appendages that contribute to

both microtubule anchoring and attachment to the plasma

membrane during ciliogenesis. The lumen of the centriole also

contains a number of internal structures. At the proximal end is a

cartwheel-like structure with nine spokes that is thought to dictate

the ninefold symmetry of the centriole (Hiraki et al., 2007;

Kitagawa et al., 2011; Nakazawa et al., 2007; van Breugel et al.,

2011). Additional electron-dense material is detected within the

distal half of the centriole lumen although this remains poorly

defined. In ciliated cells, the distal end extends towards the base

of the cilium through a region called the transition zone, which

acts as a gate to control entry of proteins to and from the cilium

(Ishikawa and Marshall, 2011).

Understanding the detailed molecular structure of the centriole

and how it is assembled has been a major challenge for cell

biologists. However, the concerted use of siRNA and

mutagenesis screens, combined with ultrastructural electron

microscopy studies, has begun to identify the roles of many

centriole components (Bettencourt-Dias and Glover, 2007;

Loncarek and Khodjakov, 2009; Nigg and Raff, 2009; Nigg

and Stearns, 2011; Strnad and Gonczy, 2008). In addition,

organelle-specific and comparative proteomics have provided

important insights into the composition of centrioles and basal

bodies (Andersen et al., 2003; Jakobsen et al., 2011; Keller et al.,

2005; Kilburn et al., 2007). Proteomic analyses of basal bodies in

Chlamydomonas and Tetrahymena revealed a set of eighteen

conserved, but previously uncharacterized, proteins, that were

termed Poc, for proteome of centriole (Keller et al., 2005;

Kilburn et al., 2007). One such protein, Poc1, has been confirmed

as a core centriole/basal body component not only in

Chlamydomonas and Tetrahymena, but also the cnidarian

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Clytia, the insect Drosophila and vertebrates, including Xenopus,zebrafish and humans (Blachon et al., 2009; Fourrage et al.,2010; Hames et al., 2008; Keller et al., 2009; Pearson et al.,

2009). Phylogenetic studies suggest the existence of Poc1proteins in all organisms that contain motile cilia at some pointin their life cycle (Woodland and Fry, 2008).

Poc1 proteins have a conserved organization consisting of an

N-terminal WD40 domain likely to form a 7-bladed b-propeller,and a C-terminal coiled-coil that includes a highly conservedsequence, known as the Poc1 motif. Separating the WD40domain and coiled-coil is a less well-conserved spacer sequence.

Human cells express two different isoforms of Poc1, termedPoc1A and Poc1B (also previously called Pix2 and Pix1,respectively), encoded by distinct genes. When overexpressed,

both Poc1 isoforms localize to centrioles in human cells withtruncation studies implicating the WD40 domain in centrosometargeting (Hames et al., 2008; Keller et al., 2009; Pearson et al.,

2009). However, it is currently unclear whether the endogenousproteins both localize at the centrosome and whether they haveredundant or distinct functions.

Studies of poc1D mutants in single-celled eukaryotes indicate

a role for Poc1 in basal body stability (Keller et al., 2009; Pearsonet al., 2009). Poc1 is also implicated in centriole length controlwith overexpression in human cells leading to elongatedcentrioles associated with centrin and c-tubulin. However,

depletion of Poc1 did not lead to shortening of centrioles inhuman cells, although mutations in Drosophila Poc1 did produceshortened spermatid centrioles (Blachon et al., 2009; Keller et al.,

2009). Consistent with a role in centriole organization, EMstudies indicate the presence of Poc1 on both the inner luminalwalls and proximal ends of centrioles, whilst in ciliated cells, it

appears at the transition zone (Hames et al., 2008; Keller et al.,2009; Pearson et al., 2009). Depletion studies suggest that Poc1B,but not Poc1A, is required for ciliogenesis in human cells, whilstPoc1 knockdown in zebrafish causes developmental defects

typical of ciliopathies (Pearson et al., 2009). Microinjection ofPoc1 antibodies in human cells also interferes with cell division(Hames et al., 2008), Together then, Poc1 proteins appear to be

important for centriole assembly and/or stability, as well asciliogenesis and cell division.

Here, using isoform-specific antibodies and RNAi depletion,we have characterized the properties and functions of the

individual human Poc1 isoforms. We find that, while bothproteins are stably incorporated into centrioles, their associationwith the centrosome is independent and exhibits distinct

dynamics. Moreover, we observed that Poc1B, but not Poc1A,is phosphorylated in mitosis, and depletion of Poc1B alone wassufficient to perturb cell proliferation. In contrast, depletion ofboth proteins, but not each one individually, led to a failure of

centriole biogenesis and defects in mitotic spindle formation.Hence, human Poc1 proteins have potentially both redundant anddistinct functions in centriole integrity and cell cycle progression.

ResultsPoc1A and Poc1B are stable components of humancentrosomes

To study the behaviour of the distinct human Poc1A and Poc1Bproteins, we first generated isoform-specific antibodies using

bacterially expressed fragments of the non-conserved spacerregions that lie between the WD40 and coiled-coil as antigens(Fig. 1A). Western blotting, combined with siRNA-mediated

depletion with two different oligonucleotides against each isoform,

indicated that these antibodies were not only capable of detectingthe endogenous proteins but were specific for the appropriateversion of Poc1 (Fig. 1B; supplementary material Fig. S1A).

Immunofluorescence microscopy of mitotic cells confirmedlocalization of both Poc1A and Poc1B at spindle poles as shownby co-localization with c-tubulin (Fig. 1C,D). Again, this stainingpattern was significantly diminished upon depletion with either of

the two different siRNAs, whereas there was no significant impacton the localization of the non-depleted isoform (Fig. 1E,F;supplementary material Fig. S1B,C). Importantly, though, we

found that loss of Poc1 staining at the two spindle poles upondepletion was not equivalent with one pole usually retaining adetectable amount of Poc1 protein (Fig. 1G,H). However,

competition with purified Poc1 proteins led to complete loss ofPoc1 staining at spindle poles attesting to the specificity of theantibodies (supplementary material Fig. S1D,E).

We hypothesized that a stable fraction of Poc1A and Poc1B that

was already incorporated into centrosomes was maintained duringdepletion, whereas incorporation into new centrosomes did not takeplace. To test this hypothesis, hTERT-RPE1 cells were depleted

with the two different siRNAs for each isoform before serumstarvation to induce ciliogenesis. This revealed that Poc1A andPoc1B remain on the basal body that subtends the axonemal

microtubules associated with the cilium, i.e. the mother centriole,but are absent from the other (Fig. 1I,J). In support of the notion thatPoc1 proteins can be stably incorporated into centrosomes, short-term siRNA-mediated depletion was not able to remove Poc1

proteins from spindle poles in cells that had previously expressedrecombinant GFP–Poc1A or GFP–Poc1B (supplementary materialFig. S2). We therefore conclude that a fraction of Poc1A and Poc1B

proteins are stably incorporated into centrioles and that depletiononly prevents their incorporation into nascent centrioles.

Poc1A and Poc1B localize to centrosomes independently

Consistent with data using recombinant proteins (Hames et al.,2008; Keller et al., 2009; Keller et al., 2005; Pearson et al., 2009),we found that endogenous Poc1A and Poc1B also localized to

interphase centrosomes and that this was independent of thepresence of intact microtubules (Fig. 2A). Analysis through thedifferent phases of mitosis revealed that Poc1A and Poc1Blocalized to spindle poles from prophase through to telophase,

while Poc1B also exhibited a weak association with spindlefibres (Fig. 2B). A previous proteomic study had raised thepossibility that Poc1A and Poc1B were present in the same

complexes and that their localization to the centrosome maytherefore be interdependent (Hutchins et al., 2010). To test thiswe examined whether the proteins were capable of interacting

with each other in cells. By transiently expressing GFP-taggedPoc1 proteins in a cell line that stably expressed TAP-taggedPoc1A, we found that recombinant Poc1 proteins can associate in

the same complexes in cells (supplementary material Fig. S3).

However, as noted above, RNAi experiments revealed thatdepletion of Poc1A did not prevent centrosome localization ofPoc1B and vice versa. We therefore performed fluorescence recovery

after photobleaching (FRAP) experiments with HeLa cells transfectedwith GFP-tagged Poc1A or Poc1B to assess their turnover rates atcentrosomes (Fig. 2C). Interestingly, we found that the two Poc1

proteins exhibited distinct dynamics with 81.8% of Poc1Aexchanging at the centrosome with a t1/2 of 7.8 seconds, whereasonly 55.1% of Poc1B exchanged within a t1/2 of 8.7 seconds. These

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dynamics were independent of the presence or absence of the other

isoform, as depletion of Poc1A did not substantially alter the

dynamics of Poc1B, and nor vice versa (Table 1). Note that a region

of interest (ROI) of 4 mm2 was chosen that allowed for the mobile

nature of the centrosomes and recovery was measured in the entire

ROI. Hence, this region will contain some cytoplasmic recovery and

Fig. 1. Poc1A and Poc1B are stable components of centrioles. (A) Schematic representation of Poc1A and Poc1B protein domain organization. WD40, WD40-

repeat domain; CC, coiled-coil domain. The red bars show the spacer regions used to generate isoform-specific Poc1 antibodies. Amino-acid positions and the

percentage of amino-acid identity of the relative domains are indicated. (B) Western blot characterization of purified Poc1A and Poc1B antibodies in HeLa cell

extracts that were either mock-treated or treated with control (siGL2), Poc1A (siPoc1A-1, siPoc1A-2) or Poc1B (siPoc1B-1, siPoc1B-2) siRNAs. Arrowheads

indicate Poc1 proteins; the asterisk indicates a non-specific protein detected. a-tubulin served as a loading control. Molecular masses (kDa) are indicated on the

left. (C,D) HeLa cells transfected with siRNAs against luciferase (siGL2) or two different siRNAs against Poc1A (C) or Poc1B (D) were fixed after 72 hours.

Cells were stained for c-tubulin (red) and Poc1A (green, C) or Poc1B (green, D). Scale bar: 10 mm. (E,F) The mean intensity of Poc1A (E) and Poc1B (F) staining

of mitotic spindle poles was quantified in cells that had been mock-depleted or depleted with Poc1A or Poc1B siRNAs, as indicated. (G,H) The percentage of cells

with 0, 1 or 2 Poc1A-positive (A) or Poc1B-positive (B) spindle poles after 72 hours RNAi treatment with GL2 or Poc1A or Poc1B siRNAs is shown (n594 for

siGL2, n560 for siPoc1A, n5100 for siPoc1B). (I,J) hTERT-RPE1 cells treated with siRNAs against GL2 or two different siRNAs against Poc1A or Poc1B

for 32 hours were subsequently serum starved for 40 hours, fixed and stained with Poc1A (I) or Poc1B (J) (green) and acetylated-tubulin (Ac-Tub; red) antibodies.

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as a result the % mobile fraction is likely to be an overestimate.

Together, then, these data support the conclusion that at least 20% of

Poc1A and 45% of Poc1B are stably incorporated into the centrosome

and would therefore be resistant to depletion.

Poc1B undergoes mitotic specific phosphorylation

During analysis of recombinant Poc1 proteins by western blot, a

doublet was sometimes observed for Poc1B. To examine whether

this might reflect a cell cycle-dependent regulation, we first

analysed the migration of recombinant TAP–Poc1 proteins in

extracts taken from asynchronous or synchronized cells. A

retarded gel mobility was apparent for TAP–Poc1B, but not

TAP–Poc1A, in extracts prepared from M-phase-, but not S-phase-,

arrested cells (Fig. 3A). A similar mitotic-specific modification

could be detected, albeit more weakly, with endogenous Poc1B, but

not Poc1A (Fig. 3B). Incubation of TAP–Poc1B isolated from

mitotic extracts with l-phosphatase led to loss of the gel-shift

indicating that Poc1B undergoes phosphorylation during mitotic

arrest (Fig. 3C). Confirmation that this is a mitotic-specific event,

rather than a consequence of microtubule depolymerisation, came

from showing that cells treated with nocodazole during an S-phase

arrest do not exhibit phosphorylation, whilst cells arrested in mitosis

with a Plk1 inhibitor rather than nocodazole show phosphorylation

(supplementary material Fig. S4A).

To determine which kinase(s) may be responsible for Poc1B

phosphorylation, we incubated MBP-tagged Poc1B with a number

of mitotic or centrosome-associated kinases. Whilst Plk1, Plk4,

Aurora A and Nek2 did not phosphorylate Poc1B in vitro,

Cdk1/cyclin B strongly phosphorylated this protein (Fig. 3D;

supplementary material Fig. S4B). Indeed, Cdk1/cyclin B

phosphorylated the full-length and spacer region of Poc1B but not

Poc1A (Fig. 3D). We also found that a tagged version of the Poc1B

spacer region could be phosphorylated in an extract prepared from

mitotically arrested cells and that this phosphorylation was blocked

by a chemical inhibitor of Cdk1, but not Plk1 or Aurora A (Fig. 3E).

Hence, we conclude that Poc1B undergoes phosphorylation in

mitotic human cells that may be mediated by Cdk1 within the non-

conserved spacer region of the protein.

Poc1B is required for cell proliferation

To explore whether human cells might have different requirements

for the two Poc1 proteins, we used the two different siRNAs to

Fig. 2. Poc1A and Poc1B localize independently to

centrosomes. (A) HeLa cells were either untreated (2 Noc) or

incubated with 500 mg/ml nocodazole (+ Noc) for 4 hours to

depolymerize microtubules before immunostaining. Methanol-

fixed cells were labelled with Poc1A or Poc1B (green) and c-

tubulin (red) antibodies. Merge panels include DNA staining

with Hoechst 33258 (blue). Magnified views of centrosomes are

shown in the insets. (B) Mitotic HeLa cells at the stages

indicated were stained with Poc1A or Poc1B (green) and c-

tubulin (red) antibodies. Merge panels include DNA staining

with Hoechst 33258 (blue). Scale bar: 10 mm. (C) HeLa cells

were transiently transfected with EGFP-Poc1A (blue diamonds)

or EGFP-Poc1B (red squares), and FRAP analysis was

performed on an asynchronous population of cells using a

confocal microscope. Recovery curves are shown on the left and

representative pseudo-coloured images from pre- and post-

bleached live samples are shown on the right. Scale bar: 1 mm.

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deplete each isoform, individually or in combination, in HeLa cells.

Analysing cell numbers at increasing times after depletion revealed

a consistent reduction in proliferation of cells treated with siRNAs

against Poc1B, but not Poc1A, as compared to control depletions

(Fig. 4A; supplementary material Fig. S5A). Combined depletion of

Poc1A and Poc1B led to a similar reduction in proliferation as

depletion of Poc1B alone. Analysis of annexin V staining revealed

no significant increase in apoptotic cells following Poc1B depletion

compared to controls indicating that reduced proliferation was not a

result of programmed cell death (data not shown). We therefore

tested whether cells were still cycling by analysing DNA content by

flow cytometry after incubating cells with nocodazole. Strikingly,

this revealed that depletion of Poc1B, but not Poc1A, reduced

nocodazole-induced accumulation of G2/M cells pointing to an

interphase delay (Fig. 4B,C). Indeed, flow cytometry analysis

revealed an increase in the fraction of G1 cells in response to Poc1B

depletion even in the absence of nocodazole (Fig. 4B,D). A similar

increase in the percentage of cells in G1 and failure to accumulate a

G2/M peak in the presence of nocodazole was found upon depletion

of Poc1B from the non-transformed cell line, hTERT-RPE1

(supplementary material Fig. S5B–D). By time-lapse imaging we

also observed a reduction in the number of Poc1B-depleted cells

entering mitosis in a 24 hour period compared to controls (Fig. 4E).

Together, these data demonstrate that loss of Poc1B, but not Poc1A,

leads to reduced proliferation and delay of cells in G1.

Co-depletion of Poc1A and Poc1B interferes with mitotic

spindle formation

We next investigated the consequences of Poc1 loss on mitotic

progression. Again, Poc1A and Poc1B were depleted either

individually or in combination from HeLa cells for 72 hours before

analysis of spindle structure in mitotic cells. Importantly, depletion of

Poc1A or Poc1B alone showed no significant mitotic defects. In

contrast, the combined depletion of Poc1A and Poc1B led to frequent

aberrations in spindle organization (Fig. 5A,B). Three major

categories of spindle defect were observed: unequal bipolar

spindles in which one half-spindle was shorter than the other,

monopolar spindles and multipolar spindles. The most prominent

phenotype was the existence of monopolar spindles, which accounted

for 38.1% of spindles in the Poc1A/Poc1B co-depleted cells versus

7.4% in controls. However, unequal bipolar spindles were also

common (24.6% in the co-depletion versus 8.8% in controls).

Multipolar spindles were found in 8.9% of the mitotic cells co-

depleted of both Poc1A and Poc1B, compared to 1.4% in the controls.

Table 1. Summary of FRAP data on Poc1A and Poc1B in asynchronous HeLa cells transiently expressing GFP–Poc1A or GFP–Poc1B

GFP–Poc1A GFP–Poc1B GFP–Poc1A (siPoc1B) GFP–Poc1B (siPoc1A)

% Mobile fraction 8168 55612 76610 56611Half-life 7.862.8 8.762.9 5.861.7 10.463.6Number of cells 12 14 15 10

A 464 mm region of interest encompassing the centrosomes was bleached and the percentage mobile fraction within a 60 second period, and half-life (t1/2,seconds) were determined using easyFRAP software.

Fig. 3. Poc1B is phosphorylated in mitosis.

(A) Extracts from asynchronous (A), hydroxyurea-

arrested (S) and nocodazole-arrested (M) HEK293

cells stably expressing TAP–Poc1A or TAP–Poc1B

were separated on phos-tag gels and western blotted

with the appropriate isoform-specific Poc1 antibody

and antibodies against cyclin B and a-tubulin.

(B) Extracts from HeLa cells prepared as in A were

western blotted with isoform-specific Poc1

antibodies or antibodies against cyclin B or GAPDH.

The Poc1B gel-shift in M-phase cells is indicated

with an asterisk. (C) Extracts from nocodazole-

arrested HEK293 cells stably expressing TAP–

Poc1B were either untreated (Input) or incubated in

phosphatase buffer with (+) or without (2) l-

phosphatase for 30 min at 30 C, before western

blotting with Poc1B and a-tubulin antibodies.

(D) Histone H1, maltose-binding protein (MBP) and

full-length (FL) or spacer fragments of MBP-tagged

Poc1A or Poc1B were used as substrates in kinase

assays with Cdk1/cyclin B for 30 min at 30 C.

Samples were analysed by SDS-PAGE, Coomassie

Blue staining (CB) and autoradiography (32P). In A–

D, molecular masses (in kDa) are indicated on the

left. (E) The His-Poc1B-spacer was incubated in

soluble mitotic HeLa cell extracts with [c-32P]ATP

in the absence (2) or presence of small molecule

inhibitors of Cdk1 (RO-3306), Plk1 (BI-2536) and

Aurora A (MLN8237) before analysis as in D.

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Staining of spindle poles with antibodies against c-tubulin revealed

that the spindle defects correlated with abnormalities in spindle pole

organization. The monopolar spindles in Poc1-depleted cells

presented with only one detectable pole instead of two. Similarly,

the unequal bipolar spindles had one strongly stained pole and one

much weaker staining pole that was present on the shorter half spindle

(Fig. 5A,C). Multipolar spindles had one strong pole and one pole

that appeared fragmented. These observations indicate that in the

majority of cells co-depleted for both Poc1A and Poc1B centrosome

maturation or integrity had failed at one pole but not the other.

Co-depletion of Poc1A and Poc1B leads to failure of

centriole biogenesis

It was notable that in cells depleted of Poc1A alone or Poc1B

alone, there was no substantial loss of c-tubulin from the

centrosome that was devoid of Poc1. Strikingly, though, we

found that whereas two strong c-tubulin signals were detected in

90% of control depleted cells, cells co-depleted for both Poc1A

and Poc1B exhibited substantial loss of c-tubulin from one

centrosome. 27% of co-depleted cells had two detectable c-

tubulin dots but had one that was at least 2-fold weaker than the

other, and 39% of cells had only one detectable c-tubulin dot

(Fig. 6A,B). A similar loss was observed for the centrosomal

marker, FOP (Yan et al., 2006) (Fig. 6C). This could be

explained by a defect in centrosome maturation specifically

leading to loss of PCM recruitment. However, staining with

antibodies against markers of centriole proximal ends, notably

Cep135, Sas-6 and C-Nap1 (Fry et al., 1998; Kleylein-Sohn et al.,

2007; Leidel et al., 2005), revealed similar losses of staining from

one centrosome, such that either only one dot was detected or one

strong and one weak dot (Fig. 6D–F). Hence, the defect was not

simply in recruitment of PCM. Indeed, the frequent detection of

Fig. 4. Depletion of Poc1B proteins leads to loss of proliferation. (A) Growth curves of HeLa cells transfected with siRNA oligonucleotides against luciferase

as control (siGL2), Poc1A, Poc1B or Poc1A+Poc1B; data are means from two separate experiments. (B) HeLa cells treated with siRNAs for 55 hrs were treated

with 0.5 mg/ml nocodazole or DMSO for 17 hours, fixed, and analysed by flow cytometry. (C) Histogram of the percentage of cells in G2/M phase; data are

means 6 s.d. from three separate experiments including that shown in B. The P-value indicates the significance of difference in the percentage of cells in G2/M in

the nocodazole-treated siGL2 versus siPoc1B samples. (D) HeLa cells were depleted with siRNAs against GL2, Poc1A or Poc1B and analysed by flow cytometry

after 72 hours. The percentage increase in G1 phase relative to siGL2 is indicated; Data are means 6 s.d. from three separate experiments. (E) HeLa cells stably

expressing GFP–a-tubulin were treated with siRNAs against GL2 or Poc1B for 48 hours and the percentage of cells entering mitosis in the subsequent 24 hour

period was determined by time-lapse imaging. n5113 for siGL2 and 100 for siPoc1B.

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one strong and one weak dot with these centriole markers

provides persuasive evidence that while co-depletion of Poc1A

and Poc1B does not block the initiation of centriole duplication,

it does interfere with assembly of nascent centrioles.

Interestingly, 8% of Poc1A and Poc1B co-depleted interphase

cells had no detectable centrosome staining at all with c-tubulin or

FOP antibodies. This is consistent with the observation of co-

depleted cells undergoing cytokinesis, with one daughter cell having

one detectable centrosome and the other with none (Fig. 6G).

Indeed, during mitosis, the daughter centriole matures into a mother

and acquires distal end appendages that can be detected with

antibodies against Cep164 (Graser et al., 2007). Staining for Cep164

revealed that in control-depleted telophase cells both emerging

daughter cells were positive for Cep164 and contain mother

centrioles, whereas co-depletion of Poc1A and Poc1B led to only

one daughter cell being positive for Cep164 (Fig. 6H). This result

confirms the time-dependent loss of mother centrioles during

division of cells lacking both Poc1A and Poc1B.

Co-depletion of Poc1A and Poc1B leads to loss of centriole

integrity and failure of daughter centriole maturation

As Poc1 proteins can associate with microtubules and localize to

the inner centriole walls, we wished to determine whether the loss

of centriole proteins upon Poc1 depletion reflects a loss of

integrity of the centriolar microtubules. For this purpose, we

analysed depleted cells using antibodies against acetylated

tubulin that directly stain centriole microtubules, as well as

antibodies against the distal end centriole marker, centrin-2.

Following control depletions, .95% of interphase cells had two

strong acetylated-tubulin dots (Fig. 7A). Of these, approximately

one-third had two centrin-2 dots (i.e. were unduplicated and in

G1), whilst two-thirds had four centrin-2 dots (i.e. were in the

process of duplicating and in S or G2). This indicates that in these

cells only the microtubules of the parental centrioles, and not the

new procentrioles, are substantially modified by acetylation.

Following Poc1 co-depletion, 41.6% of cells had only one

detectable acetylated-tubulin dot, suggesting the complete

Fig. 5. Co-depletion of Poc1A and Poc1B leads to mitotic spindle defects. (A) HeLa cells transfected with siRNAs against GL2, Poc1A and/or Poc1B were

fixed after 72 hours. Cells were stained with c-tubulin (green) and a-tubulin (red) antibodies. Merge panels include DNA staining with Hoechst 33258 (blue).

Scale bar: 6 mm. (B) The percentage of cells with the spindle structures indicated. (C) The percentage of mitotic cells with c-tubulin-positive centrosomes of the

appearances indicated. Data in B and C are means 6 s.d. from two separate experiments; n5215 for siGL2, n5218 for siPoc1A, n5210 for siPoc1B and

n5235 for siPoc1A+siPoc1B. Error bars in B and C represent s.d.

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absence of a daughter centriole. Of these, 43% contained only

one centriole as detected with centrin-2 antibodies, i.e. were

unduplicated in G1, whilst 57% had two centrioles, i.e. were

duplicated in S or G2 (Fig. 7B). We also found co-depleted cells

that had one strong and one weak acetylated-tubulin signal, i.e.

they retained some remnant of a daughter centriole, but in these

.90% had less than four centrin-2 dots indicating a failure of

duplication from at least one centriole. Hence, co-depletion of

Poc1A and Poc1B clearly interferes with generation of centrioles

with stabilized microtubules.

In line with our interphase data, when mitotic cells were

examined, almost all control depleted cells contained two

Fig. 6. Co-depletion of Poc1A and Poc1B leads to loss of centrosome proteins. HeLa cells were transfected with siRNAs against luciferase (siGL2) or

Poc1A+Poc1B, as indicated, and fixed after 72 hours. Cells were stained with c-tubulin antibodies alone (A, red), c-tubulin (red) and FOP antibodies (green;

C,G), Cep135 antibodies (green) (D), C-Nap1 (red) and Sas-6 (green; E), c-tubulin (red) and Cep164 (green; H). (B) The percentage of interphase cells with 2

strong, 1 weak and 1 strong, 1 alone or 0 c-tubulin-stained spots. Data are means 6 s.d. from two independent experiments (n5207 for siGL2 and n5222 for

siPoc1A+siPoc1B). (F) The percentage of interphase cells with 2 strong, 1 weak and 1 strong, 1 alone or 0 C-Nap1-positive centrosomes were scored with respect

to the number of Sas-6 dots seen in the same cells. Data are from a single experiment (n5100 for siGL2 and n5110 for siPoc1A+siPoc1B). Merged images

include DNA staining with Hoechst 33258 (blue). Magnified views of centrosomes, when visible, are shown. Scale bars: 6 mm (A,C,D,E,H); 11 mm (G).

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centrin-2 dots and one acetylated-tubulin dot at each pole.

However, this was not the case in Poc1A and Poc1B co-depleted

cells. Of these, ,30% had two centrin-2 dots at one pole but none

at the other, whilst 20% had two dots at one pole and one at the

other (Fig. 7C,D). Importantly, in the co-depleted cells, the

acetylated-tubulin antibodies only stained the pole containing a

pair of centrin dots, and did not stain the pole with a single

centrin dot. This strongly supports the hypothesis that cells

lacking Poc1A and Poc1B undergo one round of duplication, but

the new centrioles formed do not mature and cannot initiate a

second round of duplication in the subsequent cycle.

In order to confirm the presence of an immature centriole in

cells co-depleted for Poc1A and Poc1B, we stained cells with an

antibody against centrobin, a marker of procentrioles that is lost

Fig. 7. See next page for legend.

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once centrioles initiate duplication in S-phase (Zou et al., 2005).

In control depleted mitotic cells, both spindle poles stained

positively for c-tubulin and each of these was associated with a

strong centrobin dot, comprising the new pro-centriole within

each pole (Fig. 7E). Consistent with our previous data, though,

the majority of mitotic cells co-depleted for Poc1A and Poc1B

had only one pole that is positive for c-tubulin. However, they

frequently possessed two separated centrobin dots, only one of

which was associated with the c-tubulin-positive pole (Fig. 7E).

Hence, we propose that following the combined depletion of

Poc1A and Poc1B, cells retain one mature centriole that can

undergo duplication and recruit c-tubulin, but acquire one

immature centriole that fails to duplicate or recruit c-tubulin.

DiscussionHere, using isoform-specific antibodies combined with siRNA-

mediated depletion, we have explored the function of the two

isoforms of the centrosomal Poc1 protein expressed in human

cells. Intriguingly, our data suggest that Poc1A and Poc1B act

redundantly in centriole organization, but that Poc1B has

additional, non-redundant functions in cell cycle progression.

These conclusions are based on the fact that depletion of Poc1B,

but not Poc1A, interfered with cell proliferation, whereas

depletion of both proteins, but not either protein alone, caused

a loss of centriole integrity, which in turn led to defects in mitotic

spindle formation.

Firstly, our data confirm that both human Poc1 proteins are

core components of the centrosome throughout the cell cycle.

Although this is the first time that the localization of each isoform

at the endogenous level has been studied, this result was not a

surprise as expression of recombinant proteins had shown that

Poc1A and Poc1B could associate with the centrosome (Hames

et al., 2008; Keller et al., 2009; Keller et al., 2005; Pearson et al.,

2009). We found that, despite the ability of recombinant proteinsto dimerise, Poc1 proteins were not dependent on each other for

their localization to the centrosome. Moreover, FRAP-basedexperiments indicate that they have distinct dynamics at thecentrosome, with a higher fraction of Poc1A protein undergoingrapid exchange than Poc1B. Interestingly, a stable and dynamic

population of Poc1 was also identified in Tetrahymena, whichonly expresses one Poc1 protein (Pearson et al., 2009). However,it is worth noting that a fraction of both Poc1A and Poc1B was

stably incorporated, explaining why a signal for both proteinsremains after depletion and potentially why depletion of eachprotein alone is not sufficient to destabilize centrioles.

Functional studies in Tetrahymena first suggested a role forPoc1 in basal body stability (Pearson et al., 2009). Deletion of thePOC1 gene led to structural defects in basal bodies that interferedwith ciliogenesis and eventually led to cell death. Specifically,

although the cartwheel structure was intact, there were defects inluminal and microtubule structures at the proximal ends of basalbodies arguing strongly that Poc1 is required to correctly

assemble a basal body. Our data provide persuasive evidencethat this role is conserved in human cells, but that the presence ofeither Poc1A or Poc1B may be sufficient to fulfil this function.

Analysis of mitotic cells revealed that depletion of either proteinalone had no obvious consequence, whereas their co-depletionled to clear defects in spindle organization. The predominant

phenotypes were unequal bipolar spindles and monopolarspindles, and in both cases, this correlated with the presence ofone normal pole and one pole that either had much reduced c-tubulin or none at all. Further analysis of centriole markers led us

to conclude that combined loss of Poc1A and Poc1B leads notsimply to loss of centrosome maturation but a more profound lossof centriole integrity. The occasional observation of multipolar

spindles with one apparently fragmented pole supports the notionthat Poc1 proteins contribute to centriole stability.

Depletion of Poc1A or Poc1B led to their absence specifically

from one of the two centrosomes. This is most easily explainedby the maintenance of a stable population of Poc1 proteins in thepre-existing centrioles, but no incorporation of Poc1 into newcentrioles. We therefore hypothesize that when Poc1 proteins are

co-depleted, centrosomes are initially capable of undergoingcentriole duplication, but Poc1 proteins are not available forincorporation into nascent centrioles rendering them unstable.

When these cells enter mitosis, a relatively normal bipolardivision results. However, each offspring cell now contains onlyone stable centriole, so when these enter the next S-phase, the

stable centriole initiates duplication, but the unstable centrioledoes not. When this cell enters mitosis, it assembles an unequalbipolar or monopolar spindle due to the presence of only one

functional centrosome, or even a multipolar spindle due to afragmented centriole. And if this cell manages to complete abipolar division, then it will generate one cell with a stablecentriole, but the other cell will have only an unstable centriole

or, possibly, none at all (Fig. 7F).

The question arises as to how precisely Poc1 proteins ensurecentriole integrity. We have previously shown that Poc1 is a

microtubule-binding protein, while immuno-EM studies indicatethat a fraction of the protein localizes to the centriole lumen walls(Hames et al., 2008; Pearson et al., 2009). The loss of acetylated

tubulin from structures that still stained, albeit weakly, forcentrin-2 suggests that immature centrioles are being generatedbut that the centriolar microtubules are not getting stabilized.

Fig. 7. Co-depletion of Poc1 proteins leads to loss of centriole integrity.

(A–D) HeLa cells transfected with siRNAs against luciferase (siGL2) or

Poc1A+Poc1B were fixed after 72 hours. Cells were stained with acetylated-

tubulin (red) and centrin-2 (green) antibodies. Interphase cells are shown in A

and mitotic cells in C. (B) The percentage of interphase cells with 2 strong, 1

weak and 1 strong or only 1 acetylated-tubulin-positive centrosomes is

indicated with respect to the number of Centrin-2 dots. Data are means 6 s.d.

from two separate experiments (n5157 for siGL2 and n592 for

siPoc1A+siPoc1B). (D) The percentage of mitotic cells with 2 centrin dots at

each pole (2/2), 2 centrin dots at one pole and one at the other (2/1), 2 centrin

dots at one pole and none at the other (2/0), 1 centrin dot at each pole (1/1) or

1 centrin dot at one pole and none at the other (1/0). Data are from a single

experiment (n5176 for siGL2 and n5179 for siPoc1A+siPoc1B). (E) HeLa

cells transfected with siRNAs against luciferase (siGL2) or Poc1A+Poc1B for

72 hours were stained with antibodies against c-tubulin (red) and centrobin

(green). DNA was stained with Hoechst 33258 (blue). (F) A schematic model

showing the consequences of depleting Poc1 protein (green) on centriole

stability and duplication, together with the predicted localization of centrobin

under these conditions (pink). Upon siRNA depletion, Poc1 remains stably

associated with the parental centrioles but is not incorporated into the nascent

procentrioles, preventing their stabilization. When this cell enters mitosis, it

will produce daughter cells that have a stable mother centriole but an unstable

daughter centriole. When this cell then enters S-phase, the stable mother

centriole will initiate duplication but produce another unstable centriole,

which is incapable of duplication. When this cell arrives at the next mitosis,

the cell contains one spindle pole (pole no. 1) containing the old mother

centriole and an unstable procentriole, and one pole (pole no. 2) containing a

single unstable daughter centriole or no centriole at all. Such a cell may

generate an unequal bipolar or monopolar spindle.

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Poc1 may therefore be directly binding and stabilizing centriolarmicrotubules, in a manner similar to that recently shown forcentrobin (Gudi et al., 2011). Alternatively, it could be recruiting

other microtubule-associated proteins, including those involvedin post-translational modifications of tubulin. Finally, it could bethat the release of proteins like centrobin is required to generate

stable centrioles and this does not happen in the absence of Poc1.

It has been reported that depletion of human Poc1 proteinsprevents centrosome duplication (Keller et al., 2009). Whilst weagree that unstable centrioles lose the ability to duplicate, our databest support a model whereby the primary function of Poc1 is to

contribute to centriole stability rather than initiation of duplication.We frequently detected one strong and one weak signal with anumber of different centriole markers, including Sas-6 and

Cep135, which represent proximal end markers, and FOP, whichlocalizes more towards the distal end. This, together with thepresence of centrobin at structures that lack c-tubulin, is indicative

of an immature or unstable structure rather than no structure at all.Indeed, the presence of a strong and weak dot of C-Nap1 argues forthe presence of a correctly disengaged centriole, suggesting that

unstable centrioles can persist through one cell division event.Interestingly, overexpression of GFP–Poc1B promoted formationof abnormally long centrioles (Keller et al., 2009), which isconsistent with the notion that Poc1 not only stabilizes, but also

perhaps promotes extension of, centrioles.

Depletion of Poc1B, but not Poc1A, also interfered withproliferation of human cells. Whether this is indicative of ahypomorphic response that is potentially related to centriole

integrity is not clear. However, it is intriguing that we alsoobserved a potential difference in the mitotic regulation of Poc1Aand Poc1B. Poc1B is clearly phosphorylated in mitosis. Whether

Poc1A is also phosphorylated in mitosis is not known, but we didnot observe a gel mobility shift for Poc1A, as we did for Poc1B,and we did not detect phosphorylation of Poc1A by Cdk1 in vitro.This, together with the fact that Cdk1 phosphorylated the divergent

spacer region, supports the notion that these two proteins may bedifferentially regulated in mitosis. Sites phosphorylated in vitro,either by Cdk1 or in a mitotic extract, were mapped in Poc1B (data

not shown). However, mutation of these sites did not lead to loss ofthe gel-shift in mitotic cells and nor did it perturb centrosomelocalization, so further studies will be required to understand the

function behind this phosphorylation.

Interestingly, depletion of another core centriolar protein,Poc5, reduced proliferation of HeLa cells as a result of interphasedelay and cell death (Azimzadeh et al., 2009). Poc5 is

incorporated into the distal end of nascent centrioles and itsdepletion blocked centriole elongation. Like Poc1, Poc5 was onlystably incorporated into mature centrioles, such that Poc5-depleted cells also revealed a single spot of Poc5 staining. It is

clear then that besides the core set of centriole duplication factorsoriginally identified in C. elegans (Dammermann et al., 2004;Leidel et al., 2005; Pelletier et al., 2006), there are a set of other

important proteins that play key roles in centriole biogenesis. Thechallenge going forward will be to identify how they interact todefine and maintain the stable architecture of centrioles and basal

bodies.

Materials and MethodsPlasmid constructions

The divergent spacer regions, corresponding to amino acid residues 304–362 forPoc1A and 303–431 for Poc1B, were amplified by PCR from the previouslygenerated GFP-Poc1A and GFP-Poc1B plasmids (Hames et al., 2008), and cloned

into the pMAL-c2E vector (New England Biolabs) to produce DNA encoding N-terminal MBP-tagged Poc1 spacer peptides. For the production of TAP–Poc1A,the full-length Poc1A cDNA was cloned into a pCMV-TAP vector (Stratagene).All plasmid constructs were verified by DNA sequencing (PNACL, Leicester).

Antibody generation

For production of Poc1A and Poc1B antibodies, rabbits were immunized withbacterially expressed fragments corresponding to the divergent spacer region ofPoc1A (amino acid residues 304–362) or Poc1B (amino acid residues 303–431),fused to an N-terminal MBP tag. For affinity purification, the MBP-tagged Poc1Aand Poc1B spacer peptides were coupled to CNBr-activated Sepharose accordingto the manufacturer’s instructions (Amersham). Antisera were passed over theappropriate columns, and after extensive washing with 10 mM Tris-HCl pH 7.5followed by 10 mM Tris-HCl pH 7.5, 500 mM NaCl, specific antibodies wereeluted with 100 mM glycine pH 2.5 or 100 mM triethanolamine pH 12.5, intotubes containing neutralizing quantities of 1 M Tris-HCl pH 8.0.

Cell culture and transfection

U2OS, HeLa, HEK293, HEK293-TAP–Poc1A and HeLa-GFP–a-tubulin cellswere grown in Dulbecco’s modified Eagle’s medium (DMEM). hTERT-RPE1cells were grown in DMEM/Ham’s F12 (1:1) supplemented with 0.348% sodiumbicarbonate solution. All cells were supplemented with 10% foetal calf serum,100 IU/ml penicillin, and 100 mg/ml streptomycin (1% Pen/Strep). Cells weregrown at 37 C in a 5% CO2 atmosphere. Primary cilia formation was induced byculturing cells in serum-free medium for 40 hours. M-phase-arrested cells wereobtained by 16 hours treatment with 500 ng/ml nocodazole and collected by shakeoff. To prepare S-phase-arrested cells, 1 mM hydroxyurea was added to cells for16 hours. Synchronization was confirmed by flow cytometry. For proliferationassays, HeLa cells grown on 12-well plate for 24 hours were treated with siRNAoligonucleotides and collected every 24 hours for 4 days. Cells were trypsinisedand resuspended in 1 ml of DMEM medium. 100 ml of resuspended cells wereadded to 10 ml of ISOTON solution and counted using a CASY cells counter(Scharfe System). Transient transfections were performed with Lipofectamine2000 (Invitrogen) according to manufacturer’s instructions.

RNAi

siRNA oligonucleotides specific to Poc1A (Hs_WDR51A_2, Hs_WDR51A_6,Hs_WDR51A_4) or Poc1B (Hs_WDR51B_4, Hs_WDR51B_2, Hs_TUWD12_3)or negative control luciferase GL2 were obtained from Qiagen (Crawley, UK) andtransfected into cells at 50 nM using HiPerfect transfection reagent (Qiagen). 48 or72 hours after transfection, cells were either fixed for immunocytochemistry orprepared for western blot or flow cytometry analysis.

Fixed- and live-cell microscopy

Immunofluorescence microscopy was carried out as previously described (Hameset al., 2008). Cells grown on acid-etched glass coverslips were fixed andpermeabilised using ice-cold methanol. For visualization of centrioles with Sas-6,C-Nap1, centrin-2, acetylated tubulin and centrobin antibodies, cells grown oncoverslips were left on ice for half an hour followed by treatment with anextraction buffer (80 mM Pipes pH 6.8, 1 mM MgCl2, 1 mM EGTA, 0.5% TritonX-100) prior to fixation with ice-cold methanol at 220 C. Primary antibodies usedwere raised against Poc1A (2.7 mg/ml), Poc1B (3 mg/ml), c-tubulin (0.15 mg/ml;Sigma), a-tubulin (0.3 mg/ml; Sigma), acetylated tubulin (0.5 mg/ml; Sigma),centrin-2 (1 mg/ml; Santa Cruz), Sas-6 (2 mg/ml; Santa Cruz), FOP (1:500; giftfrom E. Nigg), Cep164 (1:1000; gift from E. Nigg), Cep135 (1 mg/ml; Abcam), C-Nap1 (0.75 mg/ml; (Fry et al., 1998) and centrobin (1:100, gift from K. Rhee).Secondary antibodies used were Alexa-Fluor-488- and Alexa-Fluor-594-conjugated goat anti-rabbit and goat anti-mouse IgGs (1 mg/ml; Sigma). Cellswere imaged using a TE300 inverted microscope (Nikon, UK) or a Leica SP5 laserscanning confocal microscope. FRAP analysis was performed on the Leica SP5laser scanning confocal microscope and analysed using easyFRAP software(Rapsomaniki et al., 2012). For time-lapse imaging, HeLa-GFP–a-tubulin cellsgrown in 6-well plate for 24 hours were treated were siRNA oligonucleotides.Images were captured once every 10 mins over a 24 hour period from 48 to72 hours after RNAi treatment using an Olympus inverted IX81 motorizedmicroscope equipped with a 406 objective and Cell‘R software. Phase contrastand GFP images were captured for three positions in each condition. Cells weremaintained at 37 C in the presence of 5% CO2 in a thermo-controlled chamberduring the time of the experiment.

Flow cytometry

To determine cell cycle distributions, cell populations were harvested asappropriate, pelleted by centrifugation and washed in 16 PBS before beingresuspended in 1 ml 70% ice-cold ethanol. Cells were maintained in ethanol at 4 Cfor a minimum of 30 min before being stained with propidium iodide. Briefly,cells were washed twice in 16 PBS to remove all traces of ethanol, andresuspended in 16 PBS supplemented with 100 mg/ml RNase A and 50 mg/ml

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propidium iodide. Cells were stained in the dark at 4 C for a minimum of 4 hours.Cells were then analysed via flow cytometry, using a FACSCantoTM II analyser

and FACSDivaTM software (BD Biosciences).

Preparation of cell extracts, SDS-PAGE and western blotting

Preparation of cell extracts, SDS-PAGE and western blotting were performed aspreviously described (Hames et al., 2008). Phos-Tag gel were prepared asdescribed (Kinoshita et al., 2006). For western blotting, primary antibodies wereused against Poc1A (2.7 mg/ml), Poc1B (3 mg/ml), a-tubulin (0.3 mg/ml; Sigma),

cyclin B (1:1000; Cell Signaling Technology), GAPDH (0.2 mg/ml; Sigma),Protein A (2 mg/ml; Sigma) and GFP (0.1 mg/ml; Abcam). Secondary antibodieswere anti-rabbit or anti-mouse horseradish peroxidase (HRP)-labelled IgGs(1:2000; Sigma).

Immunoprecipitations

For co-immunoprecipitation experiments, HEK293 cells stably expressing TAP–Poc1A were transiently transfected with GFP-tagged Poc1A or Poc1B or GFPalone. After 24 hours, soluble extracts were incubated with pre-washed IgG-Sepharose 6 Fast Flow resin (GE Healthcare) at a ratio of 5 ml beads/mg extract for

2 hours. Tubes were then centrifuged to remove the unbound supernatant and theIgG beads washed in lysis buffer. Protein complexes were eluted by boiling in 36Laemmli buffer and analysed by SDS-PAGE and western blotting.

Kinase assays

In vitro kinase assays were carried out using purified recombinant proteins,expressed in E. coli, as substrates. Briefly, 1–5 mg of the appropriate substrate was

incubated with 1 mCi of [c-32P]ATP in kinase buffer (50 mM Hepes.KOH pH 7.4;5 mM MnCl2; 5 mM b-glycerophosphate; 5 mM NaF; 4 mM ATP; 1 mM DTT)and 5 mg of the appropriate purified kinase for 30 min at 30 C. Reactions were

stopped by addition of 25 ml of sample buffer and analysed by SDS-PAGE andautoradiography.

Mitotic phosphorylation assays

Mitotic phosphorylation assays were adapted from Hutchins et al. (Hutchins et al.,2004). Briefly, M-phase-arrested HeLa cells prepared by nocodazole treatmentwere washed and collected in EBS buffer (80 mM b-glycerophosphate; 20 mM

EGTA; 15 mM MgCl2; 100 mM sucrose; 1 mM DTT and 1 mM PMSF). Onevolume of EBS buffer was added to the cell pellet, and after 30 min on ice, cellswere freeze-thawed three times and centrifuged at 16,000 g for 30 min. Thesupernatant was collected and centrifuged again at 16,000 g for 15 min. The

resulting mitotic extracts were quantified by BCA assay, aliquoted and stored inliquid nitrogen. Upon thawing, extracts were supplemented with an ATP-regenerating system consisting of 10 mM creatine phosphate, 40 mg/ml creatinekinase, and 1 mM ATP. Mitotic phosphorylation assays were carried out on 0.5–

1 mg of purified recombinant protein by incubation in 50 mM Tris-HCl pH 7.5,100 mM [c-32P]ATP, 10 mM MgCl2, 1 mM okadaic acid plus 10 mg of mitoticHeLa cell extract, for 30 min at 30 C. Reactions were stopped by addition of 25 ml

36Laemmli buffer and analysed by SDS-PAGE and autoradiography.

AcknowledgementsWe are very grateful to all members of our laboratory for usefuldiscussion. We also thank Sun Xiao Ming and Marion MacFarlane(Leicester) for help with proliferation assays, Erich Nigg (Basel) forFOP and Cep164 antibodies and Kunsoo Rhee (Seoul) for centrobinantibody.

FundingThis work was supported by the Biotechnology and BiologicalSciences Research Council [grant number BB/F010702/1 to A.M.F.and H.R.W.]. A.M.F. also acknowledges funding from TheWellcome Trust [grant number 082828] and Cancer Research UK[grant number C1420/A9363]. We are also grateful for access tofacilities within the Centre for Core Biotechnology Services.Deposited in PMC for release after 6 months.

Supplementary material available online at

http://jcs.biologists.org/lookup/suppl/doi:10.1242/jcs.111203/-/DC1

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