2. The Cytoskeleton Flashcards
what is the basic component of microfilaments
actin
what comprises actin
G-actin assembles into F-actin (reversible process)
where does ATP bind on actin
at the (-) end
true or false:
actin is polarised
true
ATP binds to actin extending the chain, what’s the formula:
G-actin + ATP -> ADP-f-actin
where does growth occur on the actin chain
at the (+) end
define critical concentration
the concentration of free ATP-G-actin at which assembly/disassembly are equal at one of the ends
what happens if:
above critical concentration
below critical concentration
- the end will grow
- the end will shrink
what are the units for critical concentration
micromolar
what is + end CC
0.12 micromolar
what is - end CC
0.6 micromolar
what is treadmilling
rate of disassembly at one end is equal to rate of assembly at the other end
what regulates assembly and disassembly
actin-binding proteins
name 3 actin-binding proteins
profilin , Cofilin, thymosin beta4
what does profilin do?
enhances exchange of ADP for ATP on G-actin
what does Cofilin do?
enhances loss of ADP-actin from the -end
= facilitates disassembly
what does thymosin beta4 do?
binds G-actin, to provide a reserve for actin when its needed for assembly
what do capping proteins do
bind to filament ends - preventing assembly and disassembly
name 2 capping proteins
CapZ and tropomodulin
what do CapZ and tropomodulin do
CapZ = binds to and stabilises the + end
tropomodulin = binds to and stabilises the - end
what does phalloidin toxin do
binds to f-actin preventing disassembly, can be used to stain actin (immunolabelling).
found in death cap mushroom
name 2 actin cross-linking proteins
fimbrin (supports microvilli)
dystrophin (muscle cell cortex)
what is dystrophin, and what happens in Duchenne muscular dystrophy
dystrophin is an adaptor protein that binds actin to cell-adhesion molecule dystroglycan
DMD = mutation impairs muscle function and shortens lifespan.
what is the function of myosin
myosin uses ATP to generate movement to transport cellular components along actin filaments
how many different types of myosin in the human body
40
what does mutation to myosin II cause
familial hypertrophic cardiomyopathy
how can you differentiate different types of myosin
all have similar head regions, but different tail regions, allowing them to move different distances
what are the main structural differences between myosin V and myosin II
myosin V has longer necks, myosin V has globular cargo binding domains at its tail.
myosin II proteins can assemble into bipolar filaments that have contractile functions
what does the neck domain do in myosin V
acts as a lever arm for the conformational change used in the power stroke
where is myosin II found
muscles
what is the role of titin
holds myosin in position
what is the role of nebulin
holds actin filaments in position
what does tropomyosin do
blocks the myosin head from interacting with actin
what happens when calcium ions are released from the SR
calcium ions interact with troponin to induce a conformation change to tropomyosin, removing it from the myosin binding site = contraction
describe the steps involved in the myosin power stroke
- ATP binds to myosin head, releasing it from actin
- hydrolysis of ATP > ADP + Pi = myosin head rotates into cocked state
- myosin head binds actin filaments
- release of P and elastic energy straightens the myosin molecule = moving the actin filament left
- ADP released, ATP binds = myosin head released
what proportion of total protein in a typical animal is actin
5%
what other process is reliant on actin, give an example
cell crawling = cell pushes forward via actin polymerisation
e.g. neutrophils migrating towards bacteria
