Dynamic Aspects of Filament Assembly

Abstract

Actin and tubulin differ from intermediate filament proteins in their mechanisms for assembly and in the properties of the polymers they form. Built from asymmetric subunits, actin filaments and microtubules have an intrinsic structural and functional polarity. The two ends of actin and tubulin polymers differ in both the assembly rate and the critical concentration for assembly. Unlike intermediate filament polymerization, their assembly is coupled to nucleotide triphosphate hydrolysis. However, hydrolysis is not absolutely required as assembly can also proceed in the presence of ADP (Cooke 1975, Kondo and Ishiwata 1976, Pollard 1974, Lal et al. 1984), or in the presence of nonhydrolyzable analogues of ATP or GTP (Cooke and Murdoch 1973, Weisenberg et al. 1976,Penningroth and Kirschner 1977). Therefore, the energy of hydrolysis does not provide energy for assembly and is not used for useful work (except, perhaps, under certain conditions: Hill 1981). What, then, might be the role of nucleotide hydrolysis during assembly? Could it be used by the cell to organize the networks of actin filaments and microtubules?