Nucleation is the rate-limiting step in polymer formation
Formins nucleate the growth of straight, unbranched filaments
Remains associated with the growing plus end as the filament elongates
Proteins that bind to the free actin subunit modify filament elongation
Thymosin binds actin subunits and prevents incorporation in the filament. Profilin does
Stathmin binds to tubulin dimers and inhibits polymerization.
Signals such as phosphorylation change the equilibrium.
Severing proteins regulate the length and kinetic behavior of actin filaments and microtubules
Katanin severs microtubules. Requires ATP.
Gelsolin severs actin filaments.
Proteins that bind along the sides of filaments can either stabilize or destabilize them
Microtubule-associated proteins (MAPs) stabilize microtubules
Different lengths of projecting domain bundles microtubules at different density.
Tau – axon
MAP2 – dendrites and cell body
Tropomyosin stabilizes actin filaments. Important for muscle contraction.
Cofilin forces the filament to twist more and become brittle. It also speeds up ADP-actin
dissociation from the minus end. Proteins that interact with filament ends can dramatically change filament dynamics
Large effect on filament dynamics with low protein amounts
Capping protein (Ca