New Aspects of the Spontaneous Polymerization of Actin in the Presence of Salts

The mechanism of salt-induced actin polymerization involves the energetically unfavorable nucleation step, followed by filament elongation by the addition of monomers. The use of a bifunctional cross-linker, N,N′-(1,4-phenylene)dimaleimide, revealed rapid formation of the so-called lower dimers (LD) in which actin monomers are arranged in an antiparallel fashion. The filament elongation phase is characterized by a gradual LD decay and an increase in the yield of “upper dimers” (UD) characteristic of F-actin. Here we have used 90° light scattering, electron microscopy, and N,N′-(1,4-phenylene)dimaleimide cross-linking to reinvestigate relationships between changes in filament morphology, LD decay, and increase in the yield of UD during filament growth in a wide range of conditions influencing the rate of the nucleation reaction. The results show irregularity and instability of filaments at early stages of polymerization under all conditions used, and suggest that an earlier documented coassembling of LD with monomeric actin contributes to the initial disordering of the filaments rather than to the nucleation of polymerization. The effects of the type of G-actin-bound divalent cation (Ca²⁺/Mg²⁺), nucleotide (ATP/ADP), and polymerizing salt on the relation between changes in filament morphology and progress in G-actin-to-F-actin transformation show that ligand-dependent alterations in G-actin conformation determine not only the nucleation rate but also the kinetics of ordering of the filament structure in the elongation phase. The time courses of changes in the yield of UD suggest that filament maturation involves cooperative propagation of “proper” interprotomer contacts. Acceleration of this process by the initially bound MgATP supports the view that the filament-destabilizing conformational changes triggered by ATP hydrolysis and P_i liberation during polymerization are constrained by the intermolecular contacts established between MgATP monomers prior to ATP hydrolysis. An important role of contacts involving the DNase-I-binding loop and the C-terminus of actin is proposed.