Cryo-EM structures of the tubulin cofactors reveal the molecular basis of alpha/beta-tubulin biogenesis

We determined the first cryo-EM structures of the tubulin cofactor complex TBC-DEG bound to an αβ-tubulin heterodimer, revealing for the first time how TBCD, TBCE, and the Arl2 GTPase form a cage-like assembly that encircles and stabilizes the αβ-tubulin dimer to regulate its biogenesis. TBC-DEG wraps around β-tubulin while TBCE extends along α-tubulin, and a companion structure with TBCC shows that TBCC engages Arl2 and TBCD to contact the αβ-tubulin intradimer interface, promoting TBCE rotation while TBCD holds β-tubulin. Together these structures reveal a spring-and-latch mechanism in which the cage captures β-tubulin, recruits α-tubulin via TBCE, and snaps the two together into an αβ dimer ready for microtubule assembly — with TBCC acting as a regulatory clamp that stabilizes the assembled dimer at the intradimer interface. Our studies provide structural evidence that tubulin cofactors act as enzymatic regulators that assemble the invariant αβ-tubulin architecture, and that TBC-DEG and TBCC regulate the polymerization competency of αβ-tubulin for microtubule formation.