Dynamic Instability of Individual Microtubules

Abstract

We have developed video microscopy methods to visualize the assembly and disas- sembly of individual microtubules at 33-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 by a 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 dissociation rate constants were 44 and 23 s -t. For both ends, the rate of tubulin dissociation equaled the rate of tubulin association at 5 lxM. The rate of rapid shortening was similar at the two ends (plus = 733 s-t; minus = 915 s-l), and did not vary with tubulin concentration. Transitions between phases were abrupt and stochas- T HE term "dynamic instability" describes microtubule assembly in which individual microtubules exhibit alternating phases of elongation and rapid shortening. Transitions between these phases are abrupt, stochastic, and infrequent in comparison to the rates of tubulin association and dissociation at the microtubule ends (17, 28). Substantial evidence has accumulated to indicate that dynamic instabil- ity is the basic mechanism of microtubule assembly in vitro (17, 28), and in both the mitotic spindle and the cyoplasmic microtubule complex (CMTC) l (10, 11, 34, 35, 37). Al- though microtubule dynamics within the cell may be regu- lated by various microtubule-associated proteins (MAPs) and other intracellular regulatory molecules, it is important 1. Abbreviations used in this paper: CMTC, cytoplasmic microtubule com- plex; DIC, differential interference contrast; MAP(s), microtubule-associ- ated protein(s). © The Rockefeller University Press, 0021-9525/88/10/1437/12 $2.00 The Journal of Cell Biology, Volume 107, October 1988 1437-1448 tic. As the tubulin concentration was increased, catas- trophe frequency decreased at both ends, and rescue frequency increased dramatically at the minus end. This resulted in fewer rapid shortening phases at higher tubulin concentrations for both ends and shorter rapid shortening phases at the minus end. At each concentration, the frequency of catas- trophe was slightly greater at the plus end, and the frequency of rescue was greater at the minus end. Our data demonstrate that microtubules assembled from pure tubulin undergo dynamic instability over a twofold range of tubulin concentrations, and that the dynamic instability of the plus and minus ends of microtubules can be significantly different. Our analysis indicates that this difference could produce treadmilling, and establishes general limits on the effectiveness of length redistribution as a measure of dynamic instability. Our results are consistent with the existence of a GTP cap during elongation, but are not consistent with existing GTP cap models.