Cytoskeletal structures

 The cytoskeleton, which consists of a network of protein
filaments extending throughout the cytoplasm of all eukaryotic and
prokaryotic cells.
Key functions of cytoskeleton:
(1) Structure and Support (2) Intracellular Transport (3) Motility
(4) determined position of organelles (5) Involved in cell division.

 The cytoskeleton is composed of three filamentous structures.
 Microtubules,
 Microfilaments,
 Intermediate Filaments together form an elaborate
interactive network.

Microtubules:
 Microtubules are rigid hollow rods approximately 25 nm in diameter.
 They function both to determine cell shape and in a variety of cell
movements, the intracellular transport of organelles, and the separation of
chromosomes during mitosis.
 Microtubules are components of a diverse array of structures, including
the mitotic spindle of dividing cells and in the core of cilia and flagella.
 Microtubules are composed of a single type of globular protein called
tubulin.
 Tubulin protein is a dimers consisting of two closely related 55 kd
polypeptides: Alpha tubulin and beta tubulin.

 When viewed in cross section, microtubules are seen to consist of 13
proto-filaments aligned side by side in a circular pattern within the
wall.
 Noncovalent interactions between adjacent protofilaments are
thought to play an important role in maintaining microtubule
structure.

 Each protofilament is made up of β-tubulin and one α-tubulin
subunit.
 The protofilament is asymmetric, with an β-tubulin at one end
and α-tubulin at the other end.
 One end of a microtubule is known as the plus end and is
terminated by a row of β-tubulin subunits .
 The opposite end is the minus end and is terminated by a row of
α-tubulin subunits.

 In particular, the GTP bound to β-tubulin is hydrolyzed to GDP
during polymerization.
 This GTP hydrolysis weakens the binding affinity of tubulin
for adjacent molecules, thereby favouring depolymerisation.

 Colchicine and colcemid are examples of commonly used
experimental drugs that bind tubulin and inhibit microtubule
polymerization, which in turn blocks mitosis.
 Two related drugs (vincristine, Taxol and vinblastine) are
used in cancer chemotherapy (Anticancer drugs) because they
selectively inhibit rapidly dividing cells.

Assembly of Microtubules:
 In animal cells, most microtubules extend outward from the
centrosome.
 During mitosis, microtubules similarly extend outward from
centrosomes to form the mitotic spindle, which is responsible for the
separation and distribution of chromosomes to daughter cells.

 Microtubules also play a key role in maintaining the internal
organization of cells.
 Treatment of cells with microtubule-disrupting drugs can seriously
affect the location of membranous organelles, including the ER
and Golgi complex.

Cilia and Flagella:
 Cilia and flagella are microtubule-based projections of the plasma
membrane that are responsible for movement of a variety of
eukaryotic cells.
 The microtubules are arranged in a characteristic "9 + 2" pattern.
 In which a central pair of microtubules is surrounded by nine
outer microtubule doublets.

Structure and Organization of Microfilaments:
 Microfilaments are approximately 8 nm in diameter and composed of
globular subunits of the protein actin.
 Microfilaments are the smallest filaments of the cytoskeleton.
 The major cytoskeletal protein of most cells is actin.
 In the presence of ATP, actin monomers polymerize to form a flexible,
helical filament.
 The terms actin filament, F-actin, and microfilament are basically
synonyms for this type of filament.
 Consequently, the two ends of an actin filament have different structures
and dynamic properties.

Assembly and Disassembly of Actin Filaments:
 Actin was first isolated from muscle cells, in which it constitutes
approximately 20% of total cell protein.
 Actin filaments involved in muscle contraction.
 Individual actin molecules are globular proteins of 375 amino acids
(43 kd).
 Each actin monomer (globular [G] actin) actin monomers
polymerize to form filaments (filamentous [F] actin).
 Each monomer is rotated by 166° in the filaments, which therefore
have the appearance of a double-stranded helix.

 Actin filaments have a distinct polarity and their ends (called
barbed or plus ends, and pointed or minus ends) are
distinguishable from one another.
 Each actin monomer has tight binding sites that mediate head-to-
tail interactions with two other actin monomers, so actin monomers
polymerize to form filaments.

 Actin filaments are then able to grow by the reversible addition of
monomers to both ends, but one end (the barbed end) elongates five
to ten times faster than the pointed end.
 The ATP associated with the actin monomer is hydrolyzed to ADP
at some time after it is incorporated into the growing actin filament.

 As long as the concentration of ATP-actin monomers remains high,
subunits will continue to be added at both ends of the filament.
 As filament elongation continues, the free monomer concentration
drops further.
 At this point, monomers continue to be added to the only plus ends
of the filaments, but a net loss of subunits occurs at their minus
end.
 This type of balance between two opposing activities is an example
of steady state.

Organization of Actin Filaments:
 Individual actin filaments are assembled into two general types
of structures called actin bundles and actin networks in the
cell.

 Actin filaments in bundles are cross-linked into parallel arrays by small
proteins called Cross linking protein (actin-bundling proteins).
 In contrast, networks are formed by Cross linking protein (actin-
bundling proteins) that cross-link orthogonal filaments.

Functions of Microfilaments:
 Microfilaments are also involved in intracellular motile
processes, such as the movement of vesicles, phagocytosis, and
cytokinesis.
 They have roles in cell movement, muscle contraction, and cell
division.

 Intermediate filaments have diameters between 8 and 11 nm, which
is intermediate between the diameters of the two other principal
elements of the cytoskeleton, actin filaments (about 7 nm) and
microtubules (about 25 nm).
 Intermediate filaments are strong, flexible rope like fibers that
provide mechanical strength to cells.
 IFs are a chemically heterogeneous group of structures that, in
humans, are encoded by approximately 70 different genes.

Intermediate Filament Assembly and Disassembly:
 The basic building block of IF assembly is thought to be a rod like
tetramer formed by two dimers (4 polypeptides) that become
aligned side by side with their N- and C-terminal pointing in
opposite (antiparallel) directions.
 Because the dimers point in opposite directions, the tetramer itself
lacks polarity.

• Intermediate filaments tend to be less sensitive to chemical agents
than other types of cytoskeletal elements and more difficult to
solubilize.
• Because of their insolubility, IFs were initially thought to be
permanent, unchanging structures

Functions of Intermediate filaments:
 Unlike the microtubules and microfilaments, the intermediate
filaments have no direct involvement in cell movement.
 They do however appear to support this mechanism by providing
mechanical strength to cells and tissues involved.
 Vimentin and Keratin Intermediate filaments have a key role in
maintaining the position of the nucleus within cells.
 They often form a ring-like network around the nucleus to hold it
in place.

Cytoskeletal structures

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Cytoskeletal structures