Active dynamics of microtubule and motor protein networks

Abstract

Cellular components have the remarkable ability to self-organize in space and time into higher order structures necessary to carry out biological functions. One example of such a structure is the spindle apparatus, a subcellular organelle composed primarily of microtubules that segregates chromosomes during cell division and ensures that each of the resulting daughter cells receives a complete copy of the genetic material. These microtubules are organized in part by molecular motor proteins that can crosslink and exert forces on microtubules using energy from ATP hydrolysis. Much is known about how individual motor proteins slide pairs of microtubules, but how motor proteins organize large collections of filaments, such as one finds in the spindle, remains unclear. How can length scales be bridged such that large length scale behaviors of the microtubule network can be understood in terms of nanometer scale motor-protein interactions? Chapter 1 provides a brief overview on two motor proteins important for spindle assembly, the role of these motor proteins during spindle self-organization, and the organization of microtubule networks more generally. Chapter 2 presents quantitative results demonstrating the active contraction of millimeter scale microtubule networks in Xenopus oocyte extracts. An active fluid model is proposed that can quantitatively explain the dynamics and internal architecture of these contracting networks, as well as how these properties vary under perturbations to sample geometry and motor concentration. Chapter 3 extends these results and provides further tests of this model by providing quantitative measurements of the effects of varying microtubule length and density on the dynamics of these networks. Chapter 4 presents a reconstitution of this bulk network contraction in a system composed of purified dynein and microtubules. Chapter 5 presents results on networks composed of purified microtubules and the motor protein XCTK2, which undergo a spontaneous bulk contraction followed by bulk extension of the network. Chapter 6 presents a summary of the conclusions of the thesis as a whole. Finally, the appendices contain a derivation of the active fluid model, Material and Methods for the thesis as a whole, and an additional chapter detailing the development and testing of a Bayesian approach to FLIM data analysis.