Invited review: the role of nucleotide triphosphate in actin and tubulin assembly and function.

Both actin and tubulin, the major proteins of the cytoskeleton, bind nucleotide triphosphate (NTP) and exhibit the phenomenon of "polymerization-coupled" NTP hydrolysis. In this report I review the nature of polymerization-coupled NTP hydrolysis, and its possible role in the cellular function of actin and tubulin. Polymerization-coupled hydrolysis may be viewed as simply reflecting differences in the NTPase activity of free subunit as compared to polymer. Making assumptions concerning the values of various rate constants, it is possible to write expressions for the effects of NTP hydrolysis on the kinetics of polymerization. The role of NTP hydrolysis may be viewed in at least three different ways: 1) Hydrolysis alters the kinetics of assembly and disassembly. This leads to a consideration of the role of subunit flow in microtubule and microfilament function. 2) Hydrolysis is an essentially irreversible step that separates the assembly and disassembly reactions. This suggests a role of NTP in the regulation of polymer content during cellular cycles of assembly and disassembly. 3) NTP may allow transient stabilization of intersubunit bonds. This suggests a role of NTP in nucleation and possible regulation of nonequilibrium states of assembly.