Protection from Free β-Tubulin by the β-Tubulin Binding Protein Rbl2p

Free β-tubulin not in heterodimers with α-tubulin can be toxic, disrupting microtubule assembly and function. We are interested in the mechanisms by which cells protect themselves from free β-tubulin. This study focused specifically on the function of Rbl2p, which, like α-tubulin, can rescue cells from free β-tubulin. In vitro studies of the mammalian homolog of Rbl2p, cofactor A, have suggested that Rbl2p/cofactor A may be involved in tubulin folding. Here we show that Rbl2p becomes essential in cells containing a modest excess of β-tubulin relative to α-tubulin. However, this essential activity of Rbl2p/cofactorA does not depend upon the reactions described by the in vitro assay. Rescue of β-tubulin toxicity requires a minimal but substoichiometric ratio of Rbl2p to β-tubulin. The data suggest that Rbl2p binds transiently to free β-tubulin, which then passes into an aggregated form that is not toxic.