Dynamic instability of individual microtubules analyzed by video light microscopy: Rate constants and transition frequencies

Abstract. We have developed video microscopy methods to visualize the assembly and disas-sembly of individual microtubules at33-ms intervals. Porcine brain tubulin, free of microtubule-associated proteins, was assembled onto axoneme fragments at 37°C, and the dynamic behavior of the plus and minus ends of microtubules was analyzed for tubulin concen-trations between 7 and 15.5 I.tM. Elongation and rapid shortening were dis-tinctly different phases. At each end, the elonga-tion phase was characterized bya second order as-sociation and a substantial first order dissociation reaction. Association rate constants were 8.9 and 4.3 ~tM-t s-t for the plus and minus ends, respectively; and the corresponding dissociatio