Practice question

Polymerization equilibrium of cytoskeletal filaments obeys critical concentration concept derived from actin and tubulin thermodynamics studies of reversible polymers. Binding constant for addition at plus end determined by on rate times free dimer concentration minus off rate dissociation constant. Critical concentration Cc equals off rate over on rate threshold where net growth zero and polymer mass constant. Cellular tubulin concentration about 10 micromolar generally above Cc for plus end promoting growth but may be near Cc for minus. When alpha beta tubulin pool diluted experimentally or sequestered by stathmin effective concentration falls below Cc net loss dominates filaments shrink GTP cap lost catastrophe initiated. Conversely increase above Cc favors nucleation and elongation increasing polymer mass. Unlike covalent polymers microtubules reversible so below Cc no new growth occurs and existing polymers depolymerize to restore pool to equilibrium. In vivo local tubulin gradients regulated by expression autoregulation and sequestration guide where microtubules assemble especially during mitosis where local concentration high near chromatin favoring nucleation.