ROLE OF 300-KDA COMPLEXES AS INTERMEDIATES IN TUBULIN FOLDING AND DIMERIZATION - CHARACTERIZATION OF A 25-KDA CYTOSOLIC PROTEIN INVOLVED INTHE GTP-DEPENDENT RELEASE OF MONOMERIC TUBULIN

beta-Tubulin synthesized in vitro in rabbit reticulocyte lysate is fou nd associated with 900 kDa complexes (C900) containing T Complex Polyp eptide 1 (TCP1), heat-shock protein (hsp) 70 and other unidentified pr oteins, with smaller 300 kDa complexes (C300) of unknown nature, in di meric association with reticulocyte alpha-tubulin and in monomeric for ms. Pulse-chase experiments indicated that production of fully functio nal beta-tubulin was preceded by its association with C900 and C300 mu ltimolecular complexes and by the appearance of beta-monomers. The hig h-molecular-mass forms appeared as intermediate products in the proces s leading to fully functional dimerizable beta-tubulin. C300-associate d tubulin can be released as beta-monomer by addition of a cofactor pr esent in reticulocyte lysate. Here a 25 kDa protein which releases tub ulin monomers from C300 has been identified and characterized. The pro tein specifically released monomers from C300, but not from C900, in a process favoured by GTP.