www.soran.edu.iq Cell and Molecular Biology Behrouz Mahmoudi
Cytoskeleton-1 1
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www.soran.edu.iq The cytoskeleton Is a dynamic 3-dimensional
structure that fills the cytoplasm, and is present in both
eukaryotic and prokaryotic cells. The cytoskeleton acts as both
muscle and skeleton, and plays a role in cell protection, cell
motility (migration), cytokinesis, intracellular transport, cell
division and the organization of the organelles within the cell
Cytoskeleton has three main structural components: microfilaments,
intermediate filaments, and microtubules. 2
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www.soran.edu.iq Actin filaments (also called microfilaments)
Monomers of the protein actin polymerize to form long, thin fibers
that are about 8 nm in diameter. Functions: provide mechanical
strength to the cell link transmembrane and cytoplasmic proteins
anchor centrosomes during mitosis generate locomotion in cells
interact with myosin to provide the force of muscular contraction
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www.soran.edu.iq Protein folding 4
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www.soran.edu.iq Microfilaments are solid rods made of a
protein known as actin When it is first produced by the cell, actin
appears in a globular form (G-actin) creating a filamentous form of
the protein (F-actin) Each microfilament exhibits polarity, the two
ends of the filament being distinctly different. This polarity
affects the growth rate of microfilaments, one end (termed the plus
end) typically assembling and disassembling faster than the other
(the minus end) Actin can hydrolyze its bound ATP to ADP + P i,
releasing P i. The actin monomer can exchange bound ADP for ATP.
The conformation of actin is different, depending on whether there
is ATP or ADP in the nucleotide-binding site 5
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www.soran.edu.iq G-actin (globular actin) with bound ATP can
polymerize, to form F- actin (filamentous actin). F-actin may
hydrolyze its bound ATP to ADP + P i and release P i. ADP release
from the filament does not occur because the cleft opening is
blocked. ADP/ATP exchange: G-actin can release ADP and bind ATP,
which is usually present in the cytosol at higher concentration
than ADP 6
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www.soran.edu.iq Filament growth at one end, designated plus
(+), exceeded that at the other end, designated minus (-). In
electron micrographs, bound myosin heads appear as arrowheads
pointing toward the negative end of the filament. Actin filaments
may undergo treadmilling, in which filament length remains
approximately constant, while actin monomers add at the (+) end and
dissociate from the (-) end. This has been monitored using brief
exposure to labeled actin monomers (pulse labeling) 7
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www.soran.edu.iq Actin filament nucleation The initial step in
the formation of an actin filament, in which actin monomers combine
to form a new filament. Nucleation is slow relative to the
subsequent addition of more monomers to extend the filament
Branched actin filaments are observed in most organelles, and
specific NPFs, such as WASP, N-WASP, WAVEs, WASH, and WHAMM, exist
for each organelle. three main classes of protein have been
identified that bypass the need for spontaneous nucleation and
promote the initiation of new filament assembly. Nucleators: are
the actin-related protein-2/3 (ARP2/3) complex, spire and formins.
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www.soran.edu.iq Capping proteins: bind at the ends of actin
filaments. Different capping proteins may either stabilize an actin
filament or promote disassembly. They may have a role in
determining filament length. For example: Tropomodulins cap the
minus end, preventing dissociation of actin monomers. CapZ capping
protein binds to the plus end, inhibiting polymerization. If actin
monomers continue to dissociate from the minus end, the actin
filament will shrink. Treadmilling of actin filaments can be
altered by profilin and ADF which generally increase and decrease
the size of actin filaments, respectively. 9
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www.soran.edu.iq Cross-linking proteins: organize actin
filaments into bundles or networks. Actin-binding domains of
several of the cross-linking proteins (e.g., filamin, a-actinin,
spectrin, dystrophin and fimbrin) are homologous. 10
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www.soran.edu.iq Some actin-binding proteins such as a-actinin,
villin and fimbrin bind actin filaments into parallel bundles.
Depending on the length of a cross- linking protein, or the
distance between actin-binding domains, actin filaments in parallel
bundles may be held close together, or may be far enough apart to
allow interaction with other proteins such as myosin Filamins:
Dimerize, through antiparallel association of their C-terminal
domains, to form V- shaped cross-linking proteins that have a
flexible shape due to hinge regions. Filamins organize actin
filaments into loose networks that give some areas of the cytosol a
gel-like consistency. 11
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www.soran.edu.iq FilGAP is a newly recognized filamin A
(FLNa)-binding RhoGTPase- activating protein. to control actin
remodeling. 12
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www.soran.edu.iq Spectrin: Is an actin-binding protein that
forms an elongated tetrameric complex having an actin-binding
domain at each end. With short actin filaments, spectrin forms a
cytoskeletal network on the cytosolic surface of the plasma
membrane of erythrocytes and some other cells. Ankyrins are a
family of adaptor proteins that mediate the attachment of integral
membrane proteins to the spectrin-actin based membrane cytoskeleton
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