actin Flashcards
3 independent protein networks in cytoskeleton
microfilaments 7-9nm
microtubules 25nm
intermediate filaments 10nm
define the cytoskeleton
a 3D filamentous protein network that provides s cell’s shape, internal organisation and functional polarity
describe the structure of a G-actin monomer
globular protein with 4 domains and 2 lobes separated by a deep cleft
ATP binding structure at the base of the cleft- binds ATP and Mg 2+
what induces the polymerisation of G actins
binding of cations induces polymerisation into F actin filaments
how can polarity be demonstrated
electron microscopy in myosin decoration experiements, excess of myosin SI is mixed with actin filaments
pointed end is (-)
barbed end is (+)
structure of F actin filament
2 strands of monomers wound around each other helically
each complete turn is 14 subunits
3 stages of actin polymerisation
nucleation, elongation, steady state
what causes the lag phase and when doesn’t it occur
lag is caused by g monomers forming a nucleus for elongation, if a small number of F actin nuclei are added to a solution the lag period can be skipped
what is property of steady state phase
no change in filament length
what happens during the elongation phase
the short oligomer rapidly increases in length due to addition of actin monomers to both ends
what does rate of addition of subunits depend on
ATP-G actin concentration in the solution
property of the rate of loss of subunits
it is not affected by ATP-G concentrations and is similar at the (+) and (-) ends
what is the critical concentration
the concentration of free G-actin at which rate of subunit addition = rate of subunit loss AT THE SAME END
how is critical concentration calculated
rate of disassembly/ rate of assembly
concentrations of free G actin at steady state
Cc+ < concentration < Cc-
what powers treadmilling and describe the mechanism
ATP hydrolysis provides power for treadmilling
when ATP-actin is added to (+) it is hydrolysed to ADP + pi
Pi is slowly released so there is ADP-actin subunits towards (-)
ADP- G protein dissociates from (-)
what is the role of profilin and mechanism
it catalyses the exchange of ADP to ATP which provides (+) a supply of ATP actin from ADP actin
- prolifin binds ADP-g actin opposite the nucleotide binding cleft
- The cleft is opened which enhances ADP loss
- This is replaced by ATP forming a ATP-profilin complex
- New actin subunit binds to (+) and prolifin dissociates
what is the role of cofilin
binds and severs ADP-actin into fragements
this generates more free (-) which enhances ADP-actin loss
the released ADP is recharged by prolifin
3 examples of structures that actin can form
microvilli, contractile stress fibres, adherens belt
function of actin binding proteins
regulate the shape of the actin cytoskeleton e.g. fimbrin in microvilli and alpha-actinin bundles actin together in contractile fibres
function of thymosin-beta 4
binds to ATP G-actin to inhibit its addition to either end of the filament, acts as a reservoir
it is in equilibrium with free ATP-G actin and can release it if needed
what does cofilin specifically bind to
old bits of F actin filaments that contain ADP
