Cell Biology: Actin, Myosin, and Microtubules

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Polymerization –plus and minus ends of actin, Kd, critical concentration, phases of growth of polymers (lag, growth, and steady-state), nucleating factors, why growth occurs preferentially at plus end.

a.High Kd and Cc= ADP + Factin lower affinity actin so dissociate

b.Lower kd and Cc= Gactin/Factin + ATP higher affinity actin so dissociate

c.Profilin and thymosin: Profilin binds to (-) end = plus end grow fast

d.Thymosin buffer + actin monomer = prevent polymerization or ATP hydrolysis.

e.Actin monomer (is above Kd) & regulates actin filament assembly.

Arp2/3 comples to make a seed (nucleate) acting growth. Can create tree-like web

Myosin II structure: motor protein makes force in skeletal contraction muscles.

a.Heavy chain with NT head (ATPase + actin) followed by long a.a responsible for dimerization. Light chain bind close to NT heads.

Mechanism of contraction by skeletal muscle (Ca2+ regulation including tropomyosin and troponin complex, how release of acetylcholine by neurons activates muscle cells, etc).

a.Sliding myosin II along actin causes muscle contraction

b.Myofibrils composed of contractile/sarcomeres(MYOSIN & actin).

c.Tropomyosin binds groove actin helix.

i.Rest= low Ca2 (Troponin 1 and T)

ii.Contract= high Ca2 (troponin C + 4 Ca2 makes troponin 1 to release actin)

d.Action potential in neurons= exocytosis of acetylcholine= Nernst Potent 0mV.

e.Depolarization membrane = down transvers tub=open voltage gate Ca2 = open Ryanodine=release Ca2 from SR into cytosol

f.Sarco/ER Ca2 ATPase= pump Ca2 into SR

Dynamic instability of microtubules (including catastrophe and rescue)

a.dynamic instability (rapid interconversion grow shrink)= GTP+tubulin higher affinity and lower critic value than GDP

i.if subunits add GTP + tubulin fast=tubulin cap= depolymerizat/catastrophe

ii.Rescue= might regain GTP bound and grow again

Microtubule= hollow tubules of 13 protofilaments= Tubulin (alpha [- end] and beta [+end] Tubulin)= bind to GTP (only beta +GTP = hydrolizes)

Organizing centers and orientation of microtubules, centrioles, centrosome

a.Microtubules are nucleated from MTOC where sigma tubulin is enriched.

i.7 copies create spiral ring = 1 protofilaments (binding –ends)

ii.microtubules emanate from centrosomes (MTOC)

iii.in centrosome are centrioles 9 L shape= SAS-6 proteins

iv.+end to cell peripheray and –end binding protein (clustering)

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