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models10–13 to test the nonlinear dynamicsunderlying the authors’ hypothesis. Klaus Hasselmann is at the Max-Planck-Institut fürMeteorologie, Bundesstrasse 55, D-20146 Hamburg,Germany.e-mail: klaus.hasselmann@dkrz.de1. Houghton, J. T. et al. (eds) Climate Change 1995: The Science of

Climate Change (Cambridge Univ. Press, 1996).

2. Corti, S., Molteni, F. & Palmer, T. N. Nature 398, 799–802

(1999).

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receptors oligomerize, they induce apopto-sis. Propagation of the death signal requiresan interaction between the intracellular tailof CD95 and the so-called death domain ofan adapter molecule called FADD/MORT1.The other half of FADD/MORT1 constitutesa ‘death-effector domain’ (DED), which isthought to interact with the DED of caspase-8 (another CED-3-like molecule), to formthe ‘death-induced signalling complex’.Close physical interaction of several caspase-8 molecules within this complex is thoughtto activate the caspases and, hence, switch onintracellular suicide.

Imai and colleagues1 now report that thisscheme is oversimplified. They have identi-fied a protein called FLASH, which interactswith the DED of caspase-8. The carboxy-ter-minal region of FLASH contains a DED-likedomain, which is responsible for the interac-tion with caspase-8. FLASH also has a regionof homology to CED-4/Apaf-1, throughwhich FLASH molecules seem to associatewith each other. The authors suggest thatFLASH is involved in CD95-induced apop-tosis (Fig. 1). First, cross-linking studiesshow that FLASH is recruited to the CD95receptor in the cell. Second, CD95-inducedapoptosis is facilitated by overexpression ofFLASH. And third, FLASH mutants (whichencode only the DED-like or CED-4-likeregion) inhibit apoptosis in a dominant-negative fashion.

So, then, could FLASH be a CED-4/Apaf-1-like protein that regulates CD95-inducedactivation of caspase-8? To many it will comeas a surprise that CD95-mediated activationrequires such a molecule, yet there areunmistakable parallels between FLASH andApaf-1/CED-4: each of them can self-associ-ate through the ‘CED-4-like’ domain; eachinteracts specifically with its caspase and canbind ATP; and each also interacts with amember of the Bcl-2 family. In the case ofFLASH, Imai et al. have found this factor tobe the adenoviral protein E1B19K, whichinhibits CD95-induced apoptosis specifical-ly at the level of caspase-8 activation4. It is,therefore, tempting to speculate that FLASHis a player in a CED-9/CED-4/CED-3-likeregulatory structure.

Some gaps remain. First, the hugeamino-terminal half of FLASH probably hasa function, yet deleting it has no apparenteffect. Second, it is unlikely that there is anyfunctional consequence when ATP binds toFLASH, because CD95-induced activationof caspase-8 does not depend on the energystatus of a cell5. Third, although E1B19Kbinding may affect FLASH during viralinfection, the CED-9-like regulation proba-bly involves a member of the mammalianBcl-2 family in uninfected cells. But no Bcl-2protein has yet been shown to modulate cas-pase-8 activation at the level of the receptor— and, thus, at the level of FLASH.

Irrespective of whether FLASH is a CED-

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756 NATURE | VOL 398 | 29 APRIL 1999 | www.nature.com

In some ways, a cell is a bit like a spy. Manyspies, at some point in their career, arefound out (and have to swallow a cyanide

pill), or they may defect and face executionby former colleagues. Likewise, every cell in amulticellular organism can commit suicide(in situations of extreme stress, for instance),or it may be killed when surrounding cellsactivate specific cell-surface receptors. Thebasic mechanism of cellular suicide is similarin organisms ranging from the nematodeworm to humans. And on page 777 of thisissue, Imai et al.1 describe a newly discovered— and completely unexpected — mam-malian protein that is involved in it.

Cellular suicide, known as programmedcell death or apoptosis, is a tightly controlledmechanism. In the nematode Caenorhabdi-tis elegans, it is dictated by three proteins2.CED-3, the ‘executioner’, belongs to a familyof killer proteases known as the caspases. Itis activated by interaction with CED-4, butinhibited by CED-9. A similar trinity has

been identified in mammals3. Here, Apaf-1(equivalent to the nematode CED-4), bindsto and activates caspase-9, one of the manymammalian CED-3 homologues. Apaf-1must itself be activated, and this occursthrough a conformational change inducedby the binding of ATP and cytochrome c.Activated Apaf-1 molecules join together(oligomerize) and, along with the associatedcaspase-9, form a protein complex dubbedthe apoptosome. This provides a platformfor caspase-9 activation. The third compo-nent of the mammalian trinity is the CED-9homologue, encoded by members of theBcl-2 family. These can block activation ofcaspase-9 by inhibiting the release ofcytochrome c or, possibly, by interactingdirectly with Apaf-1.

But mammalian cell death has an addi-tional feature that has not been identified innematodes. Mammalian cells express deathreceptors — such as the well-known CD95(APO-1/Fas) — on their surface. When these

Apoptosis

Life and death in a FLASHJan Paul Medema

Figure 1 Apoptotic pathways in the nematode and in mammals. a, In the nematode, the killer protease(or caspase; CED-3) is activated by CED-4 but inhibited by CED-9. b, In mammals there are manycaspases, one of which is activated by Apaf-1. Members of the Bcl-2 family inhibit apoptosis. There isalso a receptor-activated pathway, which is activated by signals arriving at the CD95 receptor. Imai etal.1 have discovered that FLASH interacts with an adapter molecule, FADD/MORT1, which bindsCD95, and also with the caspase. FLASH could control apoptosis at the level of caspase activation.

a b

DeathDeathDeath

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4 homologue, the biggest challenge now is towork out how — and when — it regulatesapoptosis. One hypothesis is that FLASH,much like Apaf-1/CED-4, simply facilitatescell death. Once FLASH has been recruitedto the receptor it could, by virtue of its abilityto self-associate, assemble several caspase-8molecules and provide a platform for theiractivation. Modulation of caspase-8 activa-tion is known to determine the fate of a cell.In fibroblasts, for example, the apoptosis-promoting c-myc oncogene is thought toincrease caspase-8 activation6. Conversely,c-FLIP, which inhibits CD95-inducedapoptosis in vivo, does so by blocking thisactivation7. Although Imai et al.1 detected adirect interaction between c-FLIP andFLASH, the role of FLASH in modulatingcaspase-8 is not yet known.

Finally, cells can be classified as type I ortype II depending, in part, on how efficientlythey form a death-induced signalling com-plex8. Imai and colleagues did not detect anydifference between the levels of FLASHexpression in these two types of cell. Howev-er, other mechanisms for altering the activity

of FLASH — such as sequestration or phos-phorylation — cannot be excluded. This is,in fact, an essential point, because the deathof activated peripheral T cells is suggested toinvolve a switch from type I to type II9.FLASH is an attractive candidate for control-ling this cellular suicide as well, because itmay dictate execution by ‘former colleagues’.It shouldn’t be long before we find outwhether this is the case.Jan Paul Medema is in the Department ofImmunohematology, Leiden University MedicalCenter, Albinusdreef 2a, 2333 AA Leiden, The Netherlands.e-mail: medema@mail.medfac.leidenuniv.nl

1. Imai, Y. et al. Nature 398, 777–785 (1999).

2. Gumieny, T. L., Lambie, E., Hartwieg, E., Horvitz, H. R. &

Hengartner, M. O. Development 126, 1011–1022 (1999).

3. Hengartner, M. O. Science 281, 1298–1299 (1998).

4. Perez, D. & White, E. J. Cell Biol. 141, 1255–1266

(1998).

5. Ferrai, D., Stepczynska, A., Los, M., Wesselborg, S. & Schulze-

Osthoff, K. J. Exp. Med. 188, 979–984 (1998).

6. Rohn, J. L. et al. Oncogene 22, 2811–2818 (1998).

7. Irmler, M. et al. Nature 388, 190–195 (1997).

8. Scaffidi, C. et al. EMBO J. 17, 1675–1687 (1998).

9. Scaffidi, C., Schmitz, I., Kramer, P. H. & Peter, M. E. J. Biol.

Chem. 274, 154–158 (1999).

mosomes, assembly of the mitotic spindle,attachment of chromosomes to the spindleand, ultimately, segregation of the chromo-somes to opposite poles of the cell(anaphase). After mitosis, the cell must becleaved in two, then the two daughter cellsreturn to a state (known as interphase) inwhich they are prepared for the next roundof DNA synthesis and cell duplication. Themain block to this process is the activity of acomplex containing cyclin B and the cyclin-dependent kinase-1 (Cdk1), which bothpromotes entry into mitosis and preventsexit from it.

Cells circumvent this block by inactivat-ing mitotic kinases such as cyclin B/Cdk1. Inthe budding yeast Saccharomyces cerevisiae,this is done by two overlapping mechanisms:destruction of cyclin B (or Clb in yeast),which is catalysed by the so-called anaphase-promoting complex (APC); and binding of aCDK inhibitor called Sic1 to the cyclin/Cdk1complex. After cells have segregated theirchromosomes, Sic1 accumulates and theAPC becomes activated through associationwith an activating subunit termed Cdh1(also known as Hct1)4–6. During mitosis,Cdh1 is phosphorylated — and inhibited —by cyclin B/Cdk1. It is activated only when aphosphatase called Cdc14 removes theinhibitory phosphate group7,8.

How does Cdc14 turn the tables on cyclinB/Cdk1? First, it dephosphorylates (and,hence, activates) a transcription factor calledSwi5, allowing fresh Sic1 to be transcribed(Fig. 1). This newly produced Sic1 is imme-diately targeted for degradation by Cdk1phosphorylation9,10, so the next effect ofCdc14 is to dephosphorylate Sic1 and reduceCdk1 activity11, thereby allowing Sic1 toaccumulate. Finally, as mentioned above,Cdc14 dephosphorylates Cdh1, allowing it

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NATURE | VOL 398 | 29 APRIL 1999 | www.nature.com 757

It is not often that one gets the opportunityto solve a basic problem and discover aregulatory principle in the process. But

this is precisely what is reported by Amonand co-workers1 on page 818 of this issue,and by Shuo et al.2 in Cell. They were inves-tigating how the cell regulates exit frommitosis — the final, chromosome-segrega-tion phase of the cell cycle — and re-entry

into the first phase, G1. And they havefound that the key is sequestration of aprotein phosphatase called Cdc14 in thenucleolus.

When cells enter mitosis, a series of care-fully orchestrated, irreversible events occurs(reviewed by Morgan3). Depending on theorganism, this includes breakdown of thenuclear envelope, condensation of chro-

Cell cycle

Mitotic treasures in the nucleolusJeffrey B. Bachant and Stephen J. Elledge

Figure 1 Model for regulation of Cdc14 duringthe cell cycle. Amon and colleagues1 and Shouet al.2 have found that, in G1, inactive Cdc14 issequestered in the nucleolus through itsassociation with the inhibitory factor Cfi1.After degradation of the cyclin-dependentkinase (CDK) inhibitor Sic1, mitotic forms ofCdk1 accumulate. This results in DNAsynthesis, entry into mitosis, and activation ofthe anaphase-promoting complex (APC). TheAPC catalyses destruction of the anaphaseinhibitor Pds1 to initiate anaphase13, and is alsoproposed to initiate a signal-transductioncascade that liberates Cdc14 from thenucleolus. Active Cdc14 facilitates inactivationof mitotic Cdk1 by activating Cdh1 andpromoting accumulation of Sic1. Cdk1inactivation then promotes cell division,breakdown of the spindle and return tointerphase. Phases of the cell cycle are shown inthe inset: G1, pre DNA synthesis; S, DNAsynthesis; G2, period between DNA synthesisand mitosis; M, mitosis, followed by celldivision.

M

S

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