Role of nucleotide hydrolysis in the polymerization of actin and tubulin

Here is presented a short survey of the main aspects of the involvement of nucleotide hydrolysis in the polymerization of actin and microtubules: 1) XTP hydrolysis is not tightly coupled to the polymerization process; XTP hydrolysis and phosphate release generate an unstable XDP-polymer which is maintained at steady state, in the presence of XTP, by terminal XTP-subunits; this feature can generate patterns of phase transitions of the polymer between stable and unstable conformations; 2) Interactions between subunits are involved in the mechanism of XTP hydrolysis; 3) XTP cleavage on the polymer is followed by the slow release of P_i; the structural and thermodynamic characteristics of the transient XDP-P_i-polymer may play a crucial role in the regulation of the dynamics of microtubules and actin filaments.