Role of 300 kDa complexes as intermediates in tubulin folding and dimerization: characterization of a 25 kDa cytosolic protein involved in the GTP-dependent release of monomeric tubulin.

beta-Tubulin synthesized in vitro in rabbit reticulocyte lysate is found associated with 900 kDa complexes (C900) containing T Complex Polypeptide 1 (TCP1), heat-shock protein (hsp) 70 and other unidentified proteins, with smaller 300 kDa complexes (C300) of unknown nature, in dimeric association with reticulocyte alpha-tubulin and in monomeric forms. Pulse-chase experiments indicated that production of fully functional beta-tubulin was preceded by its association with C900 and C300 multimolecular complexes and by the appearance of beta-monomers. The high-molecular-mass forms appeared as intermediate products in the process leading to fully functional dimerizable beta-tubulin. C300-associated tubulin can be released as beta-monomer by addition of a cofactor present in reticulocyte lysate. Here a 25 kDa protein which releases tubulin monomers from C300 has been identified and characterized. The protein specifically released monomers from C300, but not from C900, in a process favoured by GTP.