Tubulin folding cofactors as GTPase-activating proteins - GTP hydrolysis and the assembly of the alpha/beta-tubulin heterodimer

In vivo, many proteins must interact with molecular chaperones to attain th eir native conformation. In the case of tubulin, newly synthesized alpha- a nd beta-subunits are partially folded by cytosolic chaperonin, a double-tor oidal ATPase with homologs in all kingdoms of life and in most cellular com partments. alpha- and beta-tubulin folding intermediates are then brought t ogether by tubulin-specific chaperone proteins (named cofactors A-E) in a c ofactor-containing supercomplex with GTPase activity. Here we show that tub ulin subunit exchange can only occur by passage through this supercomplex, thus defining it as a dimer-making machine. We also show that hydrolysis of GTP by beta-tubulin in the supercomplex acts as a switch for the release o f native tubulin heterodimer. In this folding reaction and in the related r eaction of tubulin-folding cofactors with native tubulin, the cofactors beh ave as GTPase-activating proteins, stimulating the GTP-binding protein beta -tubulin to hydrolyze its GTP.