Micro Tubules, Micro Filaments & Intermediate Filaments

8/6/2019 Micro Tubules, Micro Filaments & Intermediate Filaments

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Microtubules, Microfilaments &

Intermediate Filaments

VM


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Microtubules:

First observed in the axoplasm of myelinatedNerve fibres by Robertis and Franchi (1953)

and was called as Neurotubules by them.

The exact nature of microtubules was brought into

Light by Sabatini and group (1963) by the use of

Glutaraldehyde fixative.

Occurrence: Except human erythrocyte, all cells

possess microtubules. High density exist in axons &

dendrites of nerve cells.


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In the cytoplasm of plant & animal cells, they occur

at following sites:-1. Cilia & flagella

2. Centrioles & basal bodies

3. Nerve processes

4. Mitotic apparatus

5. The cortex of meristematic plant cells

6. During lens formation & spermatogenesis

7. Some strucutures of protozoans, etc.


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Structure

Microtubules constitute a class of morphologic-

ally and chemically related filamentous rods, co-

mmon to both plants & animals.

A microtubule consists of a hollow tubules (long,

unbranched) 24-25 nm in diameter, severalmicrometers long, 6nm thick walls, with 13 sub-

units or proto-filaments.


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Consequently, the wall of microtubules consist of

13 linear or spiral filaments about 5 nm in diameterwhich in turn are composed of TUBULIN.

These pro-filaments have centre-to-centre spacing of

4.5 nm.

Based on staining techniques, microtubules

have been found to have a lumen of 14 nm width &a pro-filament structure in the wall.


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Chemical composition

Tubulin is the major chemical component

An acidic protein, mol wt 55 kD, sedimentation constant of

6S, occurs in two diff forms, -tubulin & -tubulin, each

Containing about 450 amino acids.

Both these forms of tubulin have many conserved or minutely

Differing aa sequence revealing that they have evolved from

a common ancestor


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One reason for evolutionary conserved sequences would be

that mutations are lethal and hence no such sequences are

found.

Tubulin heterodimers polymerizes into microtubules

A. Formation of protofilament:-

Free - tubulin dimers associate

longitudinally

B. Sheet assembly:-

Lateral association of protofilaments


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Sheets wrap around to form a hollow tube

Elongation by addition of subunits


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Microtubule Organizing Centre (MTOC)

The point of attachment of microtubules from where

their (from minus ends) assembly or plymerization starts

(Nucleating Centers for polymerization to begin).

MTOCs protect the minus ends

(slow growing) from

disassembly.

The plus ends terminate near

the cell margins


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MTOCs exist in Basal bodies,e.g. Chlamydomonas;

In centrioles, at the poles of dividing spindles etc.

Changes in the nucleating centers, Ca ion concentration &

Map involvement are responsible for turning on and

off of the organizing centers for microtubule assembly.


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Microtubule Associated Proteins (MAPs)

Basically two types of proteins found associating with the

Microtubules, viz.,

1. High Molecular Weight Proteins:- have mol wt. 200-

300 kDa2. Tau Proteins:- have mol. Wt. from 40-60 kDa.

Both these proteins have two domains, one with which

it attaches with microtubule and the other with whichit links microtubule to other cell components


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Assembly and disassembly of microtubules

Highly dynamic structure, constantly forming

& disappearing

Grow by reversible addition of sub-units &

hydrolysis of GTP followed by conformational

change

The assembly of microtubules is a programmed

process


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Sites of orientation are MTOCs

The quantity of polymerization is high at

metaphase & interphase and low at prophase

& anaphase

Polymerization process starts with

phosphorylation of tubulin monomers with

cAMP-dependent kinase

In a microtubule, assembly of tubulin dimers

occur at one end and disassembly occur at the

other


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Drugs such as colchicine, vinchrisitine and vinblastine

Block the tubulin dimer assembly at one end

However, the disassembly at the other end is progressing

unhindered leading to complete disorganization of

microtubule

Assembly is accompanied by GTP hydrolysis & the

process of assembly & disassembly involves Ca-ions &

action of Ca-calmodulin kinase

http://www.sci.sdsu.edu/movies/actin_myosin_gif.html


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The GTP cap model of dynamic instability

A microtubule is stable when capped with GTP

bound tubulins

A microtubule becomes unstable when cappedwith GDP tubulins


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Functions

1. Mechanical function:- shape of cell,e.g., dendrites, axns, microvilli etc

2. Morphogenesis:- during morphogenesis,

shape of the developing cells aredetermined

3.Cellular polarity & motility:- intrinsic polarit

Of the cells is determined by microtubules4.Contraction:- spindle in chromosomal mo

5.Circulation & transport:- macromolecular

transport


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Microfilaments

Made up of actin proteinsThe actin cytoskeleton looks like:-


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1. Characteristics of actin molecules

--abundant

--conserved

--isoforms

in vertebrates:

4 E-actin isoformsin muscle cells

1F- and 1K-actinin non-muscle cells

--polarity


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2. Actin structure

--globular monomer actinG-actin filamentous polymer actinF-actin

--1 actin molecule: binds 1 Mg++ & 1 ATP (or ADP) most abundant forms: G-actin/ATP, F-actin/ADP

--structure of a monomeric act

in 2 lobes (includes 4 subdomains) 1 deep cleft (ATP/Mg++ binding)

--the (-) and (+) sides of actin

(-)

(+)


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2. Actin structurecontinued

--G-actin assembles into long, helical F-actin polymers

all subunits in a filament point toward the same directionD polarity

--polarity ofF-actin:

(-) end: the side of cleft forATP

binding.

(+) end: the other side of cleftwhich contacts the

neighboring actin

subunit.


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Actin Filaments participate in a

variety of cell functions: Anchorage and movement of membrane

proteins-

filaments are distributed in 3-dimensional

networks throughout the cell

used as anchors with in specialized cell

junctions


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variety of cell functions: Formation of the structural core of

microvilli

On epithelial cells, help maintain shape of the

cell surface


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variety of cell functions: Locomotion of the cells

Achieved by the force exerted by actin

filaments by polymerization at their growingends

Used in many migrating cells, particularly ontransformed cells of invasive tumors

Cells extend processes from their surface bypushing the plasma membrane ahead of thegrowing actin filaments


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Intermediate Filaments (IF)

Found in most animals but not in plantsand fungi

Smaller than microtubules but larger thanmicrofilaments

Subunits are E-helical rods that assemble

into ropelike filaments Unlike microfilaments, IFs dont contribute

to cell motility


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Provides mechanical support for the plasma

membrane where it comes in contact with

other cells or with the extracellular matrix

Extremely stable- even after extraction with

solutions containing detergents and high

concentrations of salts


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Provides mechanical support for the plasma

membrane where it comes in contact with

other cells or with the extracellular matrix

Extremely stable- even after extraction with

solutions containing detergents and high

concentrations of salts


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IFs are broken down into 4

groups: Type I IF Prteins, e.g., Keratins

(cytokeratins)- in the epithelia

Acidic or basic

hard epithelial tissues- nails, hair, wool

Type II IF proteins (Vimentin)- most

abundant type In leukocytes, blood vessel endothelial

cells, mesenchymal cells


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IFs are divided into 4 groups:

Type III IF proteins, e.g., Neurofilaments-

neuronal axons

Extend from the cell body into the ends of

axons and dendrites

Provides structual support

Type IV IF Proteins, e.g., Lamins- foundin the nucleus


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Intermediate Filament Assembly

Assembled from a pair of helical monomers that

twist around each other to form coiled-coil dimers

Then 2 coiled-coil dimers twist around each otherto make a tetramer of 2 coiled-coil dimers

This forms the non-polarized unit of the IFs

(unlike microfilaments that are polarized)


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Diseases caused by defects in the IF

Epidermolysis bullosa simplex

Blisters form due to lack of normal bundles of keratin

filaments Alzheimers disease

Caused by changes in the neurofilaments with in brain

Alcoholic liver cirrhosis

Accumulation of keratin filaments forming inclusions

called mallory bodies in liver

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Micro Tubules, Micro Filaments & Intermediate Filaments