Mechanism of microtubule depolymerization. Correlation of rapid induced disassembly experiments with a kinetic model for endwise depolymerization

Microtubule disassembly has been studied using a rapid dilution technique (Karr, T.L., and Purich, D.L. (1979) J. Biol. Chem. 254, 10885-10888). Disassembly curves, generated by computer from the solution of series first order differential equations (see following paper), were fit to experimental data with excellent agreement when the diluted microtubules contained no microtubule-associated proteins. The rate constant for dimer release from microtubules was found to be 154 s-1. Assuming a critical tubulin concentration of 8 to 9 x 10(-6) M, the apparent bimolecular rate constant (2 x 10(7) M-1 s-1) for assembly is near the diffusion limit. It was also possible to use the rapid dilution technique for quantitatively correlating the disassembly rate to the number concentration of microtubule ends. These findings suggest that the dynamics of tubulin interactions with microtubules may be characterized in terms of an endwise depolymerization model. A re-evaluation of cold induced depolymerization kinetics (see miniprint supplement) is also fully consistent with our analysis of disassembly dynamics.