FORMATION AND FUNCTION OF THE RBL2P-BETA-TUBULIN COMPLEX

The yeast protein Rb12p suppresses the deleterious effects of excess b eta-tubulin as efficiently as does alpha-tubulin. Both in vivo and in vitro, Rbl2p forms a complex with beta-tubulin that does not contain a lpha-tubulin, thus defining a second pool of beta-tubulin in the cell, Formation of the complex depends upon the conformation of beta-tubuli n, Newly synthesized beta-tubulin can bind to Rbl2p before it binds to alpha-tubulin. Rbl2p can also bind beta-tubulin from the alpha/beta-t ubulin heterodimer, apparently by competing with alpha-tubulin. The Rb l2p-beta-tubuiin complex has a half-life of similar to 2.5 h and is le ss stable than the alpha/beta-tubulin heterodimer. The results of our experiments explain both how excess Rbl2p can rescue cells overexpress ing beta-tubulin and how it can be deleterious in a wild-type backgrou nd. They also suggest that the Rbl2p-beta-tubulin complex is part of a cellular mechanism for regulating the levels and dimerization of tubu lin chains.