Final answer:
Actin monomers do not fully polymerize due to the non-equilibrium process involving ATP-binding and hydrolysis, coupled with the regulatory role of actin-binding proteins that control polymer dynamics.
Step-by-step explanation:
The concentration of actin monomers is high in the cytosol, yet they do not polymerize completely into filaments due to several regulatory mechanisms. Essentially, the polymerization of actin is a non-equilibrium process facilitated by the binding of ATP to monomeric actin (G-actin). After ATP-bound G-actin monomers add to the plus end of an actin filament, ATP is quickly hydrolyzed to ADP, which weakens the binding between monomers and destabilizes the polymer, contributing to both growth and shrinkage of the polymer. Filament elongation occurs roughly 10 times faster at the plus end compared with the minus end, resulting in a process called treadmilling. Additionally, actin-binding proteins regulate actin dynamics, preventing spontaneous polymerization by binding to G-actin or to the ends of F-actin, thereby controlling the rate of filament assembly and disassembly.