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1 7 Cytoskeleton

The cytoskeleton contains three main types of protein fibers - microtubules, microfilaments, and intermediate filaments - that are involved in establishing cell shape, providing mechanical strength and support, intracellular transport, and cell movement. Microtubules are hollow cylinders made of tubulin subunits. Microfilaments are made of actin and are involved in cell shape changes, muscle contraction, cell motility, and cell division. Motor proteins such as dynein and myosin interact with microtubules and microfilaments, respectively, to drive the bending of cilia and flagella and muscle contraction.

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Aditya Agrawal
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102 views31 pages

1 7 Cytoskeleton

The cytoskeleton contains three main types of protein fibers - microtubules, microfilaments, and intermediate filaments - that are involved in establishing cell shape, providing mechanical strength and support, intracellular transport, and cell movement. Microtubules are hollow cylinders made of tubulin subunits. Microfilaments are made of actin and are involved in cell shape changes, muscle contraction, cell motility, and cell division. Motor proteins such as dynein and myosin interact with microtubules and microfilaments, respectively, to drive the bending of cilia and flagella and muscle contraction.

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Aditya Agrawal
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Cytoskeleton

Eucaryotic cells contain protein fibers that are involved in

- establishing cell shape


- providing mechanical strength
- cell movement
- chromosome separation
-intracellular transport of organelles
• There are three main types of fibers in the
cytoskeleton:

• microtubules (25 nm diameter),

• microfilaments (actin filament: 5-9 nm),


•
• intermediate filaments (10 nm).
Microtubules
An a,b-tubulin heterodimer is the basic structural unit of microtubules. The
heterodimer does not come apart, once formed. The a and b tubulins, which
are each about 55 kDa MW, are homologous but not identical. Each has a
nucleotide binding site. a-Tubulin has a bound molecule of GTP, that does not
hydrolyze. b-Tubulin may have bound GTP or GDP.

A microtubule is a hollow cylinder about 25 nm in diameter. Along the


microtubule axis, tubulin heterodimers are joined end-to-end to
form protofilaments, with alternating a & b subunits. Staggered assembly
of 13 protofilaments yields a helical arrangement of tubulin heterodimers in
the cylinder wall.

GTP must be bound to both a and b subunits for a tubulin heterodimer to


associate with other heterodimers to form a protofilament or microtubule.
Subunit addition brings b-tubulin that was exposed at the plus end into contact
with a-tubulin. This promotes hydrolysis of GTP bound to the now
interior b-tubulin. Pi dissociates, butb-tubulin within a microtubule cannot
exchange its bound GDP for GTP. The GTP on a-tubulin does not hydrolyze.
Microtubules
The structure of a microtubule and its subunit

The tubulin subunits assemble head-to-tail to create polar


filaments
Microtubule Motor Protein
• Cilia usually occur in large numbers on the
cell surface.
– They are about 0.25 microns in diameter and
2-20 microns long.
• There are usually just one or a few flagella
per cell.
– Flagella are the same width as cilia, but 10-200
microns long.
• In spite of their differences, both cilia and
flagella have the same ultrastructure.
– Both have a core of microtubules sheathed by
the plasma membrane.
– Nine doublets of microtubules arranged around
a pair at the center, the “9 + 2” pattern.
– Flexible “wheels” of proteins connect outer
doublets to each other and to the core.
– The outer doublets are also connected by
motor proteins.
– The cilium or flagellum is anchored in the cell
by a basal body, whose structure is identical to
a centriole.
• The bending of cilia and flagella is driven by
the arms of a motor protein, dynein.
– Addition to dynein of a phosphate group from
ATP and its removal causes conformation
changes in the protein.
– Dynein arms alternately
grab, move, and release
the outer microtubules.
– Protein cross-links limit
sliding and the force is
expressed as bending.
Microfilament

Microfilaments – actin filaments.


They are built from molecules of a globular protein – actin.
Role of Microfilament

Changes in cell shape

Muscle contraction

Cytoplasmic streaming

Cell motility

Cell division – cleavage furrow


formation
• In muscle cells, thousands of actin filaments are arranged parallel to
one another.
• Thicker filaments, composed of a motor protein, myosin, interdigitate
with the thinner actin fibers.
– Myosin molecules walk along the actin filament, pulling stacks of
actin fibers together and shortening the cell.
• In plant cells (and others), actin-myosin
interactions and sol-gel transformations
drive cytoplasmic streaming.
– This creates a circular flow of cytoplasm in the
cell.
– This speeds the distribution of materials within
the cell.

Fig. 7.21c

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