network of protein filaments that extend
throughout the cell that determine:
cell shape and polarity
tissue structure
adhesion
cell movement
intracellular movement (of
vesicles and of chromosomes)
essential to anchor cells to each other
essential to anchor cells to anchor extracellular matrix at cell junctions
structure
microfilament
composed of actin-binding proteins
called F-actin
filamentous actin or microfilament
polymers of individual actin
proteins called G-actin (globular
actin)
polarised double helix
13 actin subunits for
every complete turn
7nm diameter
5% total
protein
Growth
(1) requires ATP to be bound to the actin
monomer (G-actin)
Not very stable over time
ATP eventually hydrolyses to ADP and will
depolarise and come off at the negative end
very dyanmic
monomers can be added and removed from
both ends of the polymer
G-proteins add more rapidly to (+) end of
the filament
once incorporated, ATP is hydrolysed
to ADP
G-protein is removed more rapidly
from the (-) end of the filament
Function
provide support
Maintains the shape of cells
erythrocytes
absorption in the gut by forming an adhesion belt
microvilli in gut
detect vibration in the cochlea
In sterocillia
cells are depolarised or hyperpolarised by
deflections caused by sound; actin filaments keep
them rigid
Cell migration
modulate polymerisation dynamics and function
e.g. myosin, capping proteins, severin, etc.
required to hold synaptic vesicles close to the presynaptic membrane
Cell motility
e.g. migration of neutrophils (WBC) to sites of infection for
phagocytosis
four events
(1) cell pushes out protrusions at the front
(leading edge) ○ actin filament
polymerisation provides to force of membrane
protrusion
(2) ○ protrusions adhere to the
surface on which the cell is moving
through contact junctions; F-actin
connects to the focal adhesions to
provide a contractile force for the
cell
(3) the rest of the cell pulls against the
anchorage points to drag itself forward