Microtubule assembly is vital for many fundamental cellular processes. Current models for microtubule assembly kinetics assume that the subunit dissociation rate from a microtubule tip is independent of free subunit concentration. Total-Internal-Reflection-Fluorescence (TIRF) microscopy experiments and data from a laser tweezers assay that measures in vitro microtubule assembly with nanometer resolution, provides evidence that the subunit dissociation rate from a microtubule tip increases as the free subunit concentration increases. These data are consistent with a two-dimensional model for microtubule assembly, and are explained by a shift in microtubule tip structure from a relatively blunt shape at low free concentrations to relatively tapered at high free concentrations. We find that because both the association and the dissociation rates increase at higher free subunit concentrations, the kinetics of microtubule assembly are an order-of-magnitude higher than currently estimated in the literature.
Bibliographical noteFunding Information:
The authors thank Dominique Seetapun and Dr. Chris Gell for technical assistance. We thank Aleksey Demchouk for providing the microtubule tip tracking code. The LLC-PK1α strain was kindly provided by Dr. Pat Wadsworth. This work was supported by National Institutes of Health (NIH) GM076177 to A.J.H. and D.J.O, NIH GM071522 to D.J.O, and National Science Foundation (NSF) 615568 to D.J.O. M.K.G. was supported in part by a Whitaker International Scholar Fellowship. The collaboration was supported by the Institute for Mathematics and Its Applications (IMA) at the University of Minnesota.