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Summary MBOC Chapter 16 - The Cytoskeleton
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Summary of Chapter 16 of Molecular Biology of the Cell
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Chapter 16: The Cytoskeleton cytoskeleton = remarkable system of filaments
o give shape and structure to the cell
Three families of protein filaments:
1) actin filaments determine shape of the cell’s surface and are necessary for
whole-cell locomotion + drive pinching of one cell into two
o 8 nm
2) microtubules determine positions of membrane-enclosed organelles, direct
intracellular transport, and form the mitotic spindle that segregates chromosomes
during cell division
o long, hollow cylinders made of the protein tubulin
o 25 nm
3) intermediate filaments provide mechanical strength
o rope like fibres
o give elasticity and flexibility to higher animal cells
o 10 nm
motor proteins can convert the
energy of ATP hydrolysis into
mechanical force that can either
move organelles along the filaments
or move the filaments themselves
Cytoskeletal filaments adapt to form
dynamic or stable structures
The cytoskeleton determines
cellular organisation and polarity
o apical surface = upper
surface
bundled actin
filaments increase
the cell surface area
available for
absorbing nutrients
from food
o basolateral surface = lower
surface
organisation of the cytoskeleton in
polarized epithelial cells
Cell builds cytoskeletal filaments by
assembling large numbers of the
small subunits
o small subunits can diffuse
rapidly in the cytosol
o actin filaments – actin
subunits
o microtubules – tubulin
subunits
o subunits bind head-to-tail
Microtubules are built of 13
protofilaments – linear strings of
subunits joined end-to-end – that
, associate with one another laterally to form a hollow cylinder provide strength and
adaptability
o single protofilament: thermally unstable
o multiple protofilaments: thermally stable
breaking requires a lot of energy
even if you remove a small part of it, it still remains stable
Actin and tubulin bind and hydrolyse nucleoside triphosphates (ATP and GTP
respectively)
Accessory proteins and motors regulate cytoskeletal filaments
motor proteins bind to polarized cytoskeletal filament and use the energy derived
from repeated cycles of ATP hydrolysis to move along it
Bacteria contain homologs of all three of the eukaryotic
cytoskeletal filaments
Nearly all bacteria and many archaea contain FtsZ, which
can polymerize into filaments and assemble into the Z-ring
at the site where the septum forms during cell division
Actin and actin-binding proteins
Actin subunits assemble head-to-tail to create flexible, polar filaments
Actin monomer has a nucleotide, ATP or ADP, bound in a deep cleft in the centre of
the monomer
slower-growing minus end and a faster growing plus end
o because of asymmetric actin subunits
o subunits are positioned with their nucleotide-binding cleft directed toward the
minus end
o arrow part: - = pointed end ; non-arrow part + = barbed end
o persistence length = the minimum filament length at which random thermal
fluctuations are likely to cause it to bend
nucleation = subunits must assemble into an initial aggregate, or nucleus, that is
stabilized by multiple subunit-subunit contacts and can then elongate rapidly by
addition of more subunits actin filament
actin subunits oligomers growing actin filament until certain saturation point
steady state (equilibrium phase) ; actin filaments with subunits coming on and off
o nucleation (lag phase) elongation (growth phase) steady state
(equilibrium phase)
Cell can use
free energy
released
during
spontaneous
filament
polymerization
to move an
attached load
When subunits are incorporated into filaments, the hydrolysis of ANP is accelerated
o filaments: T-form & D-form
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