Lesson 27: Cytoskeleton Actin Key Players

actin (microfilaments)

shape the cell surface, drive crawling, pinch the cell in two

intermediate filaments

rope-like fibers that give mechanical strength and resist stress

microtubules (tubulin)

stiff tracks for organelle positioning, transport, and mitotic spindle

nucleation

forms a stable nucleus, slow lag, the initial step

Brownian ratchet

thermal motion opens a tiny gap between the filament tip and the membrane, a subunit fills it, and the membrane can’t slip back

Arp2/3

nucleates a filament and stays on the minus end, leaving the plus end free to grow fast, makes branches

NPF

activates Arp2/3 and works best docked on the side of an existing filament, launching a branch at 70 angle

formin

nucleates unbranched filaments that other proteins bundle into parallel cables, a dimer with an actin-binding side on each subunit, nucleates by capturing two monomers, rides the fast-growing plus endpe

profilin

binds monomer opposite the ATP cleft, blocking the minus-end side while leaving the plus-end side site open so profilin-actin adds only at plus ends

thymosin

sequesters monomers and competes with profilin, which sets how much actin is free to polymerize

cofilin

twists a filament tighter until it fragments

gelsolin

Cat2+ activated; wedges into a gap to sever, then caps the new plus end

caps

decide when a filament stops

coats

decide which proteins can touch it

cross-linker spacing

decides whether a bundle can contract