Summary MBOC Chapter 16 - The Cytoskeleton

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