The polymerization of actin: Thermodynamics near the polymerization line.

Studies of the dependence of actin polymerization on thermodynamic parameters are important for understanding processes in living systems, where actin polymerization and depolymerization are crucial to cell structure and movement. We report measurements of the extent of polymerization, F, of rabbit muscle actin as a function of temperature [T=(0 - 35)°C], initial G-actin concentration [[G[SUB0]]=(1 - 3) mg/ml], and initiating salt concentration [[KCl]=(5-15) mmol/l with bound Ca[SUP2+]], in H[SUB2]O and D[SUB2]O buffers and in the presence of adenosine triphosphate (ATP). A preliminary account of the data and analysis for H[SUB2]O buffers has appeared previously [P. S. Niranjan, J. G. Forbes, S. C. Greer, J. Dudowicz, K. F. Freed, and J. F. Douglas, J. Chem. Phys. 114, 10573 (2001)]. We describe the details of the studies for H[SUB2]O buffers, together with new data and analysis for D[SUB2]O buffers. The measurements show a maximum in φ(T) for H[SUB2]O buffers and D[SUB2]O buffers. For H[SUB2]O buffers, T[SUBp] decreases as either [G[SUB0]] or [KCl] increases. For D[SUB2]O buffers, T[SUBp] decreases as [KCl] increases, but T[SUBp] is not monotonic in [G[SUB0]]. The measurements are interpreted in terms of a Flory-Huggins-type lattice model that includes the essential steps: monomer activation, dimerization of activated species, and propagation of trimers to higher order polymers. The competition between monomer activation and chain propagation leads to the observed nonmonotonic variation of φ(T). The actin polymerization in D[SUB2]O buffer differs considerably from that in the H[SUB2]O buffer and underscores the significant deuterium effect on hydrophobic interactions and hydrogen bonding in the polymerization process. [ABSTRACT FROM AUTHOR]