Thermodynamic regulation of actin polymerization

A Flory-Huggins-type lattice model of actin polymerization under equilibriu m conditions is employed to analyze new spectroscopic measurements for the extent of actin polymerization Phi as a function of temperature T, salt con centration [KCl], and the initial concentration of actin monomers [G(0)]. T he theory subsumes existing mechanisms for actin monomer initiation, dimeri zation, and chain propagation. The extent of polymerization Phi increases w ith T to an unanticipated maximum, and the calculations explain this unusua l effect as arising from a competition between monomer activation, which di minishes upon heating, and propagating chain growth, which increases upon h eating. The actin polymerization is described as a rounded phase transition , and the associated polymerization temperature T-p depends strongly, but n early linearly on [G(0)] and [KCl] over the concentration regimes investiga ted. Our findings support the suggestion that physicochemical changes can c omplement regulatory proteins in controlling actin polymerization in living systems. (C) 2001 American Institute of Physics.