Key Concepts of Section 17.2

• The rate-limiting step in actin filament assembly is the formation of a short actin oligomer (nucleus) that can then be elongated into filaments.

• The critical concentration (C_c) is the concentration of free G-actin at which the addition of monomers to a filament end is balanced by loss from that end.

• When the concentration of G-actin is above the C_c, the filament end will grow; when it is less than the C_c, the filament will shrink (see Figure 17-8).

• ATP–G-actin is added much faster at the (+) end than at the (−) end, resulting in a lower critical concentration at the (+) end than at the (−) end.

• At steady state, actin subunits treadmill through a filament. ATP-actin is added at the (+) end, ATP is then hydrolyzed to ADP and P_i, P_i is lost, and ADP-actin dissociates from the (−) end.

• The length and rate of turnover of actin filaments is regulated by specialized actin-binding proteins (see Figure 17-11). Profilin enhances the exchange of ADP for ATP on G-actin; cofilin enhances the rate of loss of ADP-actin from the filament (−) end, and thymosin-β₄ binds G-actin to provide reserve actin when it is needed. Capping proteins bind to filament ends, blocking assembly and disassembly.