THE CYTOSKELETON
DARIYUS KABRAJI & ABHIJEET
SHEJWAL
MSC 1 BATCH 2 ROLL NO 18 & 15

INTRODUCTION
• The cytoskeleton is an integral cell component
present in all cells (except most bacterial species)
• It is the basic unit of the cell maintaining its
structural integrity
• While the Cytoplasm is the semi-fluid substance
that fills the space between the nucleus and the
cell membrane that holds all of the organelles in
suspension, the Cytoskeleton is what holds the
organelles in place, or moves them and aids in
cell movement

BASICS
• The cytoskeleton is made up of a network of
fibres composed of proteins within a cell's
cytoplasm.
• The term "cytoskeleton" refers to the filamentous
structures themselves, which are composed of a
set of proteins, each with their own shape and
designation
• Although it appears to be fixed, it is actually an
ever-changing dynamic configuration, with
components regularly destroyed, repaired or
newly formed

BASICS

FUNCTIONS
• It imparts the cell shape
• It provides protection by resistance to deformation
• It can actively contract, deforming the cell and it's
environment, allowing cells to migrate
• It is involved in cell signalling pathways
• It separates chromosomes during cell division
• It provides a blueprint to organize the organelles in
cytoplasm and for the construction of a cell wall
• It gives rise to structures such as flagella and cilia

THE EUKARYOTIC
CYTOSKELETON
• Prokaryotic cytoskeleton is a recent discovery
& unwise choice for a presentation
• Eukaryotic cytoskeleton is made up of three
structures, each with their own design and
function:
1. Microfilaments
2. Intermediate filaments
3. Microtubules

THE EUKARYOTIC
CYTOSKELETON

MICROFILAMENTS
• Monomers of the protein actin polymerize to
form long, thin fibres i.e. G-actin polymerising
to form F-actin filaments
• These are about 7 nm in diameter and, being
the thinnest of the cytoskeletal filaments, are
also called microfilaments
• They represent the active or motile part of the
cytoskeleton

MICROFILAMENTS
FORMATION OF ACTIN POLYMERS

CHARACTERISTICS
• Found in the cortical regions of the cell beneath the plasma
membrane
• Extend into cell processes involving movement
• The concentration of actin in the body’s non-muscle cells
add up to 10% of net protein
• There are three types of actin filaments- α, β, & γ of which
only α is found in muscle cells
• In non- muscle cells, actin binds to heavy myosin which
results in an arrow shaped pattern, indicating the existence
of polarity in actin, crucial to mediating cell movement
• Heavy myosin (HMM) is thus used to identify and localize
microfilaments

SENSITIVITY
• Microfilaments are sensitive to Cytochalasin-
B, an alkaloid
• This compound impairs cell activities like
• Heartbeat
• Cell migration
• Cytokinesis
• Endocytosis
• Exocytosis

SENSITIVITY

THE DYNAMIC ASSEMBLY
• Unlike other polymers such as DNA, bound
together with covalent bonds, the monomers of
actin filaments are assembled by weaker bonds.
• These bonds give the advantage that the filament
ends can easily release or incorporate monomers.
• This means that the filaments can be rapidly
remodelled and can change cellular structure in
response to an environmental stimulus.

THE DYNAMIC ASSEMBLY

FUNCTIONS
• Forms a band just beneath the plasma membrane that
provides mechanical strength to the cell
• Links transmembrane proteins to cytoplasmic proteins
• Separates dividing animal cells during cytokinesis
• Generates cytoplasmic streaming in some cells
• Generates locomotion in cells such as white blood cells
and the amoeba by giving rise to pseudopodia
• Interacts with myosin filaments in skeletal muscle
fibres to provide the force of muscular contraction
• Also involved in movement associated with furrow
formation in cell division
• Responsible for cytoplasmic streaming in plant cells
• Cell migration during embryonic development

INTERMEDIATE FILAMENTS
• Tough protein fibres in higher eukaryotic cells
• Mostly between 8-10 nm in diameter
• Named ‘intermediate’ for their moderate size
between smaller actin filaments and massive
microtubules
• In animal cells they are stable structures which
compensate for the absence of cell walls
• They can also be anchored between membranes
for extra support

INTERMEDIATE FILAMENTS

STRUCTURE
• In a cross-section, intermediary filaments have a
tubular appearance
• Each tube is made up of 4 or 5 parallel protofilaments
• They are comprised of polypeptides from 40-130 Kilo-
Daltons
• All IF proteins have a single common central region of
around 310 amino acid residues forming extended
helixes with short interruptions. The domains at the
terminal ends (amino & carboxyl) are non helical and
vary in size and sequence per type

STRUCTURE

TYPES OF IFs
• There are 4 types of intermediary filaments, each
with its own mass and function:
• Type 1: Acidic, neutral & basic Keratin (40-70 Kd)
in epidermal derivatives and epithelial cells
• Type 2: Vimentin (53 Kd), Desmin (52 Kd),
Synemin (230 Kd), Glial filaments (45 Kd) in
muscle and Glial cells
• Type 3: Neurofilament proteins (60-130 Kd) in
Neurons
• Type 4: Nuclear lamins (65-75 Kd) in Nuclear
lamellae

FUNCTIONS
• Main function involves mechanical support to the
cell
• The nucleus is guarded by an IF mesh network
that extends to the cell boundaries
• Neurofilaments protect neurons from mechanical
damage caused by the animal’s movement
• Filaments in epithelia form a tough network to
avoid incursion from external factors
• Desmin filaments support sacromeres in muscle
cells
• Vimentin filaments surround and guard fat cell
droplets

MICROTUBULES
- ABHIJEET SHEJWAL
• First observed in axoplasm of myelinated
nerve fibers by Robertis and Franchi 1953.
• Microtubules of plant cells observed in 1963
by Ledbetter and Porter.
• Found in all eukaryotes, either free in
cytoplasm or form part of centriole, cilia and
flagella
• High densities of microtubule in axon &
dendrites of nerve cells.

STRUCTURE
• Rigid hollow rods, approx. 25 nm in diameter.
• Single type of globular protein Tubulin M.W.- 55 kD
• Tubulin contains 2 dimers, α & β Tubulin. Each contain
450 amino acids.
• Tubulin dimers polymerize to form microtubule
• 13 linear protofilament around hollow core.
• Polar structure of microtubules have 2 ends:
a) Fast growing plus end
b) Slow growing minus end
• Polarity helps determine direction of movement.
• Undergo continuous assembly & disassembly in
structure.
• Initiation of microtubule assembly is done by γ-tubulin
in centrosome.

STRUCTURE
• Minus end of cytoplasmic microtubule bound to MTOCs from which
polymerization starts.
• Plus end is protected by capping protein to avoid disassembly.
• MAPs associate protein with surface of microtubule
• MAPs of 2 types:
a) HMW proteins (M.W. 200 to 300 D)
b) tau proteins (M.W. 40 to 60 D)
• MTOCs initiation of assembly, serve template for polymerization,
exists in basal bodies, in centriole.
• ON-OFF of MTOCs regulated by changes in nucleation center,
changes in Ca 2+ concentration, involvement of MAPs.

FORMATION
• Tubulin dimers depolymerize as well as polymerizes, α & β tubulin
bind to GTP to regulate polymerization.
• GTP bound to β tubulin is hydrolyzed to GDP during
polymerization.
• GTP hydrolysis weakens binding affinity of tubulin which favours
depolymerization.
• Tubulin bound to GDP are lost from minus end while replaced by
addition of tubulin bound to GTP at plus end continuously.
• GTP Cap is observed at plus end which leads to continuous growth
of microtubule.
• Dynamic instability described by Tim Mitchisan & Marc Kirschner
in 1984.
• Rapid turnover of microtubule is imp for remodeling of
cytoskeleton occurs during mitosis.

FORMATION
• Polymerized tubulins are high at interphase,
metaphase while low at prophase, anaphase.
• Phosphorylation of tubulin monomer by cAMP
kinase
• Assembly inhibited by drugs like
colchicine,vinblastine, vincristine
• In vivo control of polymerization involves Ca 2+
and calcium binding protein calmodulin.
• King 1986 in vitro assembly of microtubule occur
in presence of low Ca 2+ concentration, MAPs ,
GTP, level of free tubulin monomers.

FUNCTION
• Determine cell shape.
• Determine variety of cell movements (cell
locomotion, intracellular transport of
organelles, separation of chromosomes during
mitosis)
• Determine intrinsic polarity of cells and
motility
• Contraction of spindle

APPLICATION
• Drugs affect microtubule assembly are useful
in treatment of cancer. e.g.
i. Colchicine & Colcemid – binds tubulin,
inhibit polymerisation which blocks mitosis.
ii. Vin cristine & vin blastine – selectively inhibit
rapidly dividing cells.
iii. Taxol – stabilizes microtubule, blocks cell
division. Used as anticancer agent.

FUTURE PROSPECTS
• Advisable to study the cytoskeleton in
prokaryotes since so little is known about them
• Probably more undiscovered functions carried
out by the cytoskeleton, might be interesting field
of specialization
• Diseases or disorders of any kind concerning
these components
• Basic formation and destruction pathways of the
components