REVISITING MOLECULAR MECHANISMS OF MICROTUBULE ASSEMBLY, DISSASEMBLY AND FORCE PRODUCTION

Orateur : Nikita GUDIMCHUK (Lomonosov Moscow State University, Russia)

Microtubules are essential cytoskeletal polymers in all eukaryotic cells. Thanks to their dynamic instability, microtubules efficiently search and capture chromosomes in mitosis. Upon association with chromosomebound kinetochore proteins, microtubule tips produce pulling and pushing forces that aid in the accurate segregation of sister chromatids. Understanding of the mechanisms that underlie these processes has been limited by our lack of knowledge of microtubule tip structure and the conformational changes responsible for microtubule switches between assembly and disassembly. In contrast to most previous models, our recent descriptions of microtubule tips by electron tomography in vivo (six species) and in vitro (under several experimental conditions) found that the ends of these dynamic polymers display flaring protofilaments in both growing and shrinking states (McIntosh et al., 2018). Curved protofilament morphology at microtubule ends may have profound implications for microtubule dynamics, for mechanical force generation, and for the regulation of both processes by associated proteins. However, existing models for microtubule dynamics have not yet explicitly considered this structural feature, nor have they employed data from microtubule force generation experiments. We have used a Brownian dynamics method to construct and systematically analyze a comprehensive model for microtubule dynamics and force production. We demonstrate that a description of microtubule assembly and disassembly with flared ends can be achieved under a range of conditions, using simple tubulin lateral interaction energy potentials. However, force generation experiments put constraints on model parameters, pointing to the presence of a high and steep activation energy barrier in the lateral tubulin interaction energy potential. When properly constrained, the model describes the development of large pulling forces by shortening microtubules and considerable pushing forces by growing microtubules. Moreover, the flared morphology of growing microtubule tips enables sustained assisting forces, mediated via a circular kinetochore coupler. A loaddependent acceleration of microtubule growth rates provides an explanation for the long-standing problem of synchronizing the assembly and disassembly of microtubules connected to opposite spindle poles during metaphase chromosome oscillations.

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