intermediate filaments and micro tubules
TRANSCRIPT
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by,
VANKAYALA DEEPA SATISH
(11MBT0017)
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Introduction
cytoskeleton
intracellular transport and cellular
division.Eukaryotes Microfilaments
Intermediate filament Microtubules
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Microfilament
Thinnest filaments of the cytoskeleton. Made up of actin subunits.
Microfilaments help to generate the forcesused in cellular contraction and basic cell
movements. Enable a dividing cell to pinch off into two
cells and are involved in amoeboidmovements of certain types
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Actin filaments
It is monomer of single polypeptide chain.
It has a nucleotide binding site i.e ATP orADP.
Flexible than microtubules.
The mechanism of filament treadmillingand dynamic instability similar to that of
microtubule(tubulin)
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Intermediate filaments
Intermediate filaments (IFs) are a family ofrelated proteins that share common structuraland sequence features.
Average diameter of 10 nanometes
Most types of intermediate filaments arecytoplasmic, but one type, the lamins, arenuclear.
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Structure
The domain structure of IF isconserved.
Each protien has a globular domain at
the N and C terminiwhich surrounds thealpha-helical rod domain
The dimer is formed through the
interaction of rod domain to form acoiled coil.
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Cytoplasmic IF assemble into non-polar unit-length filaments (ULF), which then assembleinto longer structures. Part of the assemblyprocess includes a compaction step, in whichULF tighten and assume a smaller diameter.
The reasons for this compaction are not wellunderstood, and IF are routinely observed tohave diameters ranging between 6 and 12nm.
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The N and C terminals of each filament arealigned.
Some Intermediate filaments form
homodimers; other form heterodimers.Also, as opposed to actin or tubulin,
intermediate filaments do not contain abinding site for a nucleoside triphosphate.
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Cytoplasmic IF do not undergo treadmillinglike microtubules and actin fibers, but they
are dynamic.
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Biomechanical properties
IFs are rather deformable proteins thatcan be stretched several times their initiallength.
The key to facilitate this large deformationis due to their hierarchical structure, whichfacilitates a cascaded activation ofdeformation mechanisms at different
levels of strain.
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INTRODUCTION
Microtubules
Cylindrical stiff tubes 24 nm across
Polymers of globular proteins - tubulins
Dynamic system - assembly anddisassembly
Found in all eukaryotes
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MICROTUBULES - FUNCTION
Support for cellular components andtracks for
several motor proteins
Responsible for variety of movements
Beating of cilia and flagella
Transport of membrane vesicles
Extension of neuronal growth cone
Formation of mitotic spindle
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Polarized
Plus and minus ends
Have radial organization
Microtubule organizing centercentrosome or basal body
Stability controlled by microtubule-associated proteins (MAPs)
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Tubulin
Heterodimer of two different
subunits - and
Products of separate genes but highlyhomologous
and subunits rarely dissociate
subunit - not included in the filament,has a role in microtubule assembly
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The a & b tubulins, each about 55 kDa, arehomologous but not identical. Each has anucleotide binding site.
a-Tubulin has a bound GTP, that does not hydrolyze.
b-Tubulin may have bound GTP or GDP.
Under certain conditions, b-tubulin can hydrolyze itsbound GTP to GDP plus Pi, release Pi, andexchange the GDP for GTP.
ab
GTP GTP
tubulinheterodimer
An a,b-tubulin heterodimeris the basic structural unit of
microtubules.The heterodimer does notcome apart, once formed.
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A microtubule is a hollowcylinder, about 24 nm indiameter.
Along the microtubule axis,tubulin heterodimers join end-to-end to form protofilaments, with
alternating a & b subunits.Staggered assembly of 13protofilaments yields a helicalarrangement of tubulin
heterodimers in the cylinderwall.
seam
microtubule3-starthelix
b-GDPa-GTP
b-GTP
a-GTP
+
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This results in the wall having a "seam" where,instead of the predominant aa & bb lateral contacts,a subunits are laterally adjacent to b.
Electron microscopy ofmicrotubules decorated withmotor protein heads indicate a
"3-start helix.
Each turn of the helix spans 3tubulin monomers (e.g., a,b,
a).
seam
microtubule3-starthelix
b-GDPa-GTP
b-GTPa-GTP
+
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seam
microtubule3-starthelix
b-GDPa-GTP
b-GTP
a-GTP
+
During in vitro microtubuleassembly, heterodimers joinend-to-end to formprotofilaments.
These associate laterally to formsheets, & eventually
microtubules.Heterodimers can add ordissociate at either end of amicrotubule in vitro, but there is
greatertendency for subunits toadd at the plus end, where
b-tubulin is exposed.
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As with actin filaments, microtubulescan undergo treadmilling, with:
addition of tubulin heterodimers at the
plus end dissociation of tubulin heterodimers at
the minus end.
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GTP must be bound to both a & b subunits for atubulin heterodimer to associate with other
heterodimers to form a protofilament or microtubule.Subunit addition brings b-tubulin that was exposedat the plus end into contact with a-tubulin.
This promotes hydrolysis of GTP bound to the nowinterior b-tubulin. Pi dissociates, but b-tubulin withina microtubule cannot exchange its bound GDP forGTP.
The GTP on a-tubulin does not hydrolyze.
abab
GTP GDP GTP GTP
protofilament() (+)
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References
GENE VIII, BENZAMIN LEWIS ,2004 EDITION Aldaz H, Rice LM, Stearns T, Agard DA.
"Insights into microtubule nucleation from thecrystal structure of human gamma-tubulin."Nature. 2005 May 26;435(7041):523-7.
Murphy, S.M, Preble, A.M., Patel, U.K.,O'Connell, K.L., Dias, D.P., Moritz, M., Agard,D.A., Stults, J.T., and Stearns, T., GCP5 andGCP6: Two New Members of the Humangamma -Tubulin Complex, Mol. Biol.Cell(2001)12 3340-3352,
THE CELL,BRUCE ALBERTS
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THANK YOU