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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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Actin Filaments participate in a
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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Actin Filaments participate in a
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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Intermediate Filaments (IF)
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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Intermediate Filaments (IF)
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