intermediate filaments and micro tubules

8/2/2019 Intermediate Filaments and Micro Tubules

1/24

by,

VANKAYALA DEEPA SATISH

(11MBT0017)


2/24

Introduction

cytoskeleton

intracellular transport and cellular

division.Eukaryotes Microfilaments

Intermediate filament Microtubules


3/24


4/24

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


5/24

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)


6/24

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.


7/24

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.


8/24

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.


9/24

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.


10/24

Cytoplasmic IF do not undergo treadmillinglike microtubules and actin fibers, but they

are dynamic.


11/24

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.


12/24

INTRODUCTION

Microtubules

Cylindrical stiff tubes 24 nm across

Polymers of globular proteins - tubulins

Dynamic system - assembly anddisassembly

Found in all eukaryotes


13/24

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


14/24

Polarized

Plus and minus ends

Have radial organization

Microtubule organizing centercentrosome or basal body

Stability controlled by microtubule-associated proteins (MAPs)


15/24

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


16/24

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.


17/24

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

+


18/24

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


b-GDPa-GTP

b-GTPa-GTP

+


19/24

seam


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.


20/24

As with actin filaments, microtubulescan undergo treadmilling, with:

addition of tubulin heterodimers at the

plus end dissociation of tubulin heterodimers at

the minus end.


21/24

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() (+)


22/24


23/24

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


24/24

THANK YOU

intermediate filaments and micro tubules

Documents