The structure of the TBCE/TBCB chaperones and α-tubulin complex shows a tubulin dimer dissociation mechanism

Abstract

Tubulin proteostasis is regulated by a group of molecular chaperones termed tubulin cofactors (TBC). Whereas tubulin heterodimer formation is well-characterized biochemically, its dissociation pathway is not clearly understood. We carried out biochemical assays to dissect the role of human TBCE and TBCB chaperones in αβ-tubulin dissociation. We used electron microscopy and image processing to determine the three-dimensional structure of human TBCE, TBCB and the α-tubulin (αEB) complex, which is formed via αβ-tubulin heterodimer dissociation by the two chaperones. Docking the atomic structures of domains of these proteins, including the TBCE UBL domain as we determined by X-ray crystallography, allowed description of the molecular architecture of the αEB complex. We found that heterodimer dissociation is an energy-independent process that takes place through disruption of the α/β-tubulin interface caused by a steric interaction between β-tubulin and the TBCE CAP-Gly (cytoskeleton-associated protein glycine-rich) and LRR (leucine-rich repeat) domains. The protruding arrangement of chaperone UBL (ubiquitin-like) domains in the αEB complex suggests direct interaction of this complex with the proteasome, thus mediating α–tubulin degradation.