■ Within cells, the actin cytoskeleton is dynamic, with filaments able to grow and shrink rapidly.
■ Polymerization of G-actin in vitro is marked by a lag period during which nucleation occurs. Eventually, a polymerization reaction reaches a steady state in which the rates of addition and loss of subunits are equal (see Figure 19-10).
■ The concentration of actin monomers in equilibrium with actin filaments is the critical concentration (Cc ). At a G-actin concentration above Cc, there is net growth of filaments; at concentrations below Cc, there is net depoly-merization of filaments.
■ Actin filaments grow considerably faster at their (+) end than at their (—) end, and the Cc for monomer addition to the (+) end is lower than that for addition at the (—) end.
■ The assembly, length, and stability of actin filaments are controlled by specialized actin-binding proteins that can sever filaments or cap the ends or both. These proteins are in turn regulated by various mechanisms.
■ The complementary actions of thymosin p4 and profilin are critical to regulating the actin cytoskeleton near the cell membrane (see Figure 19-10).
■ The regulated polymerization of actin can generate forces that move certain bacteria and viruses or cause changes in cell shape.
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