what does actin form
microfilaments
where is actin most likely present
cortex, below the plasma membrane
what are the plural forms of actin
lamellipodia and filopodia (F-actin)
what is the monomer of actin
G-actin
How does actin exibit polarity
has a minus and plus end - show speed of shrinkage or growth
What is the function of nucleations
allows for growth
What happens in the step of nucleation?
the lag state - 3 G-actins form a nucleus (trimer)
Slow step forms the nucleus
What happens in the elongation state?
the growth phase - actin filament - rapid growth once nucleus formed
What happens in the steady state?
the equilibrium phase - subunits being added and subtracted at equal rates
rate of association = rate of dissociation (Cc)
What is the significance of the plus and minus end of actin?
(-) end has a higher critical concentration
growth and shrinkage can occur on both ends
What is treadmilling?
growth on one end, and shrinkage on the other = no change in length
both ends are constantly dynamic
How is the ATP cap formed?
when the rate of G-actin added is greater than the rate of ATP hydrolysis
- means constant growth
what form of ATP when added?
ATP added
ADP removed
What are the three toxins for actin
Latrunculin - depolymerizes
Cytochalasin B - depolumerizes
Phalloidin - stabilizes (unable to disassemble)
What are the three toxins for microtubules
Taxol - stabilizes (unable to disassemble)
Nocodazole - depolymerizes
Colchicine - depolymerizes
What are the 2 proteins that modify G-actin?
Thymosin- inhibit addition of G-actin to bind on plus end
Profilin - favors addition of G-actin to bind to plus end
What are the 2 examples of an actin nucleator?
Arp 2/3: (branched actin) caps the minus end so only growth occurs on the plus end
Formin: (unbranched actin) see-saw back and forth - bound to plus end and adding G-proteins on the plus end
What is the function of CapZ?
prevents growth and shrinkage on the plus end (cap)
What is the function of the tropomodulin?
prevents growth and shrinkage on the minus end (cap)
what is tropomodulin?
a type of capping protein
Where can capping proteins be found?
skeletal muscle cells
what is the significance of these actin filaments being capped?
this ensures only sliding is occurring between the different actin forms (no shrinking or growing)
What is the function of cofilin?
an actin severing protein - forces filament to twist tightly until breaks - allows for broken actin to be recycled and added to the top as ATP
What are the 2 actin bundling proteins?
fimbrin - shorter protein, holds actin closer together
Alpha-actinin - longer protein, holds actin farther apart
What type of bundling protein is used for a contractile bundle?
alpha - actinin (need more space to but motor protein between them)
what type of bundling protein is used for a tight parallel bundle?
fimbrin (prevents myosin II from entering bundle)
What is the motor protein for actin?
myosin
What is the structure of the actin motor protein that creates the contractive properties?
skeletal muscle myosin II - 2 heavy chains and 4 light chains - in a coiled coil (alpha-helical domain)
- flower like ends that join their stems creating a bare zone in the middle
what is the function of active myosin?
motor fixed, filament is not
1. in rigor state (fixed) myosin head bound to actin filament
2. ATP molecule binds to myosin head (loses affinity to actin filament)
3. myosin head hydrolyzes ATP and in cocked position
4. myosin head loses inorganic phosphate and has high affinity to bind to actin filament
5. loses ADP and changes conformation - in straight position and slides the space to a G-actin)
when is the myosin II muscle contraction active?
Ca2+ released
What are the functions of the Z-discs?
there are 2 z-discs on both ends of a sarcomere.
this creates the contractile unit of a skeletal muscle cell that shortens length by sliding
What are the 2 proteins that regulate skeletal muscle contraction?
Tropomyosin - binds filament groove, covering the myosin binding site (no nerve signal bc in the way)
Troponin - binds with Ca2+ and pulls tropomyosin out of myosin binding site so contraction can occur (nerve receives signal)
What activates myosin II contraction?
phosphorylation of the light chains
(starts in a folded inactive state, then gets phosphorylated and changes confirmation to form a baby filament)
What does the barbed end and the pointed end mean in actin filament?
pointed end - plus side
barbed end - minus side
What end is the critical concentration point always higher?
minus end
what does it mean if you have a ATP cap?
that actin is actively growing
How does formin grab G-actins?
by working with profilin which is a protein that favors the addition of G-proteins on the plus end
Where do we find branched actin?
in the cortex
Where do we find unbranched actin?
in sweeping/ crawling
- stress fibers, filopodia (finger-like projection on the surface of cells), and contractile ring
what is the cortex
in side the cell near the membrane that helps the cell with structural support and shape