Electron Cryotomography Provides Evidence of a Novel Pathway for Tubulin’s Polymerization into Microtubules

Richard McIntosh, Ph.D.
Distinguished Professor Emeritus
University of Colorado Boulder, Department of Physics
12:00 PM – Friday, October 4, 2019

The McIntosh lab has used electron tomography to examine the tips of microtubules (MTs) growing in vivo and from pure tubulin in vitro. The ends of all these elongating polymers display bent protofilaments that curve away from the MT axis, in contrast to previous reports which showed tips that were either blunt or sheet-like. Neither high pressure nor rapid plunge-freezing seems to be responsible for the protofilament curvatures we see. The curvatures of protofilaments on growing and shortening MTs are similar; all are most curved at their tips, suggesting that GTP tubulin in solution is bent and must straighten to be incorporated into the microtubule wall. Variations in curvature suggest that protofilaments are flexible in their plane of bending but rigid to bending out of that plane. Modeling by Brownian dynamics suggests that protofilament straightening for MT growth can be achieved by thermal motions, providing a simple mechanism with which to understand tubulin polymerization. They have tested their model by comparing predictions about the dependence of MT growth rates on the concentrations of soluble tubulin, of polymerization slowing drugs, and of protein catalysis that enhance growth; all predictions agree nicely with experimental data. This model also accounts for the raggedness at the tips of growing but not shortening polymers and for the abilities of growing and shortening MTs to exert force. Why are the shapes of a growing MT tips that we see so different from the shapes previously described? Richard will explore this issue with ideas about both the strength of tomographic imaging and the limitations of rapid freezing to capture labile biological structures.