LAUSR

Disclosing the metastable nature of microtubules (MTs), long and hollow tubes formed by αβ-tubulin heterodimers, has been a long-standing problem for cell scientists. MTs are essential for cargo transport and provide mechanical forces in chromosome segregation. Both MT assembly and disassembly proceed via subtle changes in the shape of tubulins and the accumulation of mechanical tension, either allosterically or fueled by GTP hydrolysis, ultimately driving the MT lattice beyond the stability threshold. However, it is still elusive how and where these shape changes contribute to the life cycle of a MT. In our work, we aim at addressing two specific questions: (a) why does GTP-tubulin polymerize and GDPtubulin does not?; (b) how and where does GTP hydrolysis destabilize the MT lattice? We study the conformational dynamics of both unassembled and MT-integrated tubulin by a combination of cryo-EM and sub-millisecond atomistic simulations. We first demonstrate how GTP binding by tubulin is linked to its dynamics and energetics in solution and why GTP-induced flexibility is crucial to MT assembly [1]. Using accurate atomistic models refined against near-atomic cryo-EM data [2], we then show that tubulin dimers locked in the MT lattice accumulate strain upon GTP hydrolysis that is amplified by lateral dimer-dimer interactions, suggesting that tubulin operates as a 'loadable spring'. Taken together, a model emerges in which MT (dis)assembly is not exclusively driven by the nucleotidedependent dynamics of individual tubulins, but more generally, by their strongly non-additive collective behavior. [1] Igaev M. and H. Grubmüller, 2018. Microtubule assembly governed by tubulin allosteric gain in flexibility and lattice induced fit. eLife 7:e34353. [2] Igaev M. et al., 2019. Automated cryo-EM structure refinement using correlation-driven molecular dynamics. eLife 8:e43542 Friday, December 13, 2019, 13:00 Room PH 127 Contact: Hendrik Dietz, [email protected], phone: 089-289-11615