A Novel Step in (beta)-Tubulin Folding Is Important for Heterodimer Formation in Saccharomyces cerevisiae

Undimerized (beta)-tubulin is toxic in the yeast S. cerevisiae. It can arise if levels of (beta)-tubulin and (alpha)-tubulin are unbalanced or if the tubulin heterodimer dissociates. We are using the toxicity of (beta)-tubulin to understand early steps in microtubule morphogenesis. We find that deletion of PLP1 suppresses toxic (beta)-tubulin formed by disparate levels of (alpha)- and (beta)-tubulin. That suppression occurs either when (alpha)-tubulin is modestly underexpressed relative to (beta)-tubulin or when (beta)-tubulin is inducibly and strongly overexpressed. Plp1p does not affect tubulin expression. Instead, a significant proportion of the undimerized (beta)-tubulin in plp1(delta) cells is less toxic than that in wild-type cells. It is also less able to combine with (alpha)-tubulin to form a heterodimer. As a result, plp1(delta) cells have lower levels of heterodimer. Importantly, plp1(delta) cells that also lack Pac10, a component of the GimC/PFD complex, are even less affected by free (beta)-tubulin. Our results suggest that Plp1p defines a novel early step in (beta)-tubulin folding.