Mitosis and microtuble assembly.

Microtubules reconstituted in vitro are identical in helical structure with tubules making up the mitotic spindle and both are formed by a helical condensation-polymerization mechanism. The protomer of microtubules is a heterodimer (alphabeta), of mol.wt. 110000 and sedimentation coefficient, SO25,w, 6S. This dimer has one binding site for colchicine or podophyllotoxin, two sites for Vinca alkaloids, two sites for guanine nucleotides and Ca2+-binding sites. There is also a beta-chain phosphoserine. Modulation of these properties is discussed as a possible way of regulating the competence of tubulin to polymerize. Reconstituted microtubules depolymerize to a mixture of 30-36S oligomeric tubulin and 6S dimer molecules. The 30-36S tubulin appears as a ring or disc when made visible in the electron microscope by negative staining. Three pathways of microtubule assembly have been proposed involving this ring as an intermediate: (1) the uncoiling of these rings into protofilaments; (2) the stacking of rings into macrotubules; (3) the rings as a scaffold for the assembly of a short segment of microtubule helix. Finally, the regulation of mitosis is discussed in terms of recent studies of tubulin and its polymerization, and studies in vitro and in vivo of the process of mitotic-spindle formation and disassembly.