INCORPORATION OF TUBULIN SUBUNITS INTO DIMERS REQUIRES GTP HYDROLYSIS

A toroid multisubunit complex of 800-900 kDa has been implicated in as sisting protein folding of at least two cytoplasmic proteins, actin an d tubulin. This process is dependent on the presence of magnesium ions and ATP hydrolysis. In vitro translation of cDNAs encoding different alpha- and beta-tubulin isotypes also gives rise to the formation of c omplexes of about 300 kDa. These complexes have been functionally impl icated in the incorporation of tubulin monomers within the tubulin het erodimer. This work shows that, in addition to ATP hydrolysis, the inc orporation of newly synthesized tubulin subunits into functional heter odimers requires GTP hydrolysis in the presence of magnesium ions. A t wo-step process is suggested, a first ATP-dependent step in which the 900 kDa complexes are implicated in a similar way to the step taking p lace in actin folding, and a second GTP-dependent step in which the 30 0 kDa complexes are involved in the assembly of the heterodimer.