Formation and Function of the Rbl2p–β-Tubulin Complex

ABSTRACT

The yeast protein Rbl2p suppresses the deleterious effects of excess β-tubulin as efficiently as does α-tubulin. Both in vivo and in vitro, Rbl2p forms a complex with β-tubulin that does not contain α-tubulin, thus defining a second pool of β-tubulin in the cell. Formation of the complex depends upon the conformation of β-tubulin. Newly synthesized β-tubulin can bind to Rbl2p before it binds to α-tubulin. Rbl2p can also bind β-tubulin from the α/β-tubulin heterodimer, apparently by competing with α-tubulin. The Rbl2p–β-tubulin complex has a half-life of ∼2.5 h and is less stable than the α/β-tubulin heterodimer. The results of our experiments explain both how excess Rbl2p can rescue cells overexpressing β-tubulin and how it can be deleterious in a wild-type background. They also suggest that the Rbl2p–β-tubulin complex is part of a cellular mechanism for regulating the levels and dimerization of tubulin chains.

ACKNOWLEDGMENTS

We thank R. Williams (Vanderbilt University) and M. Caplow (University of North Carolina) for discussions of heterodimer dissociation and the members of our laboratory for critical contributions.

J.E.A. was supported in part by an NSF predoctoral fellowship. L.R.V. was supported in part by a predoctoral fellowship from HHMI. This work was supported by a grant from the NIH to F.S.