A structural mechano-chemical model of microtubule nucleation.

Abstract

Microtubules (MTs) are the backbone of cytoskeleton and crucial for many essential processes, such as mitosis, morphogenesis and migration. A critical step in the assembly and regulation of cellular MT network is nucleation. There are two types of templates for MT nucleation in cells: existing MT and γ-tubulin ring complex. Both types of nucleations have been reconstituted in vitro, but their mechanisms remain poorly understood. Here, we developed a physically rigorous structure-based mechano-chemical model that provides an unified mechanism for both types of nucleations. For nucleation from seed MTs, the key observations are: 1) the critical tubulin concentration for nucleation is much higher than that for growth, 2) the concentration-dependent time lags. Our model shows that the bottleneck for nucleation is frequent catastrophes (i.e., the switching from MT growth to shortening) that keep removing nascent MTs from the seed, which is reduced as tubulin concentration increases. The observed time lag is due to repeated assembly and removal of nascent MTs, which delay nucleation. For nucleation from γ-tubulin ring, the most intriguing question is the effects of ring closure, a complex-wide conformational change required for the ring complex to match the tube-like geometry of MT. Our model shows that ring closure helps tubulins to first form a sheet, which is then closed into tube with the help of the ring closure process. Because catastrophe cannot develop while sheet is open, MTs more likely grow longer, increasing the chance of nucleation. Thus, ring closure allows more regulated and efficient MT nucleation.