Role of α-tubulin helix 11' in heterodimer conformation and microtubule dynamics

Tubulin heterodimers transition from a curved conformation in solution to straight conformation when assembled into the microtubule lattice. Many proteins and small molecules alter microtubule dynamics by binding and stabilizing the curved or straight conformations; however, we have a poor understanding of the regions of the tubulin heterodimer that contribute to this transition and whether these conformations represent sources of phenotypic variation in human disease and across species. We previously identified a role for α-tubulin helix 11' (H11') in the curved-to-straight transition. Using computational simulations and tubulin mutants in budding yeast, we show that H11' variants associated with human disease destabilize the curved conformation of the heterodimer and disrupt α-tubulin function in cells. Comparing α-tubulin sequences across eukaryotes demonstrates strong conservation of H11' with the exception of several α-tubulin isotypes expressed during mitosis in the amoebae Naegleria fowleri and Naegleria gruberi. Introducing H11' sequence variants from Naegleria in budding yeast α-tubulin increases heterodimer exchange at microtubule plus ends and destabilizes mitotic spindles. We provide evidence that interactions between α-tubulin H11' and β-tubulin H8 stabilize the curved conformation of tubulin heterodimers, and that the equilibrium between curved and straight conformations is an ancient feature of tubulin evolution.