Modulation of Tubulin Polypeptide Ratios by the Yeast Protein Pac10p

Abstract

Abstract Normal assembly and function of microtubules require maintenance of the proper levels of several proteins, including the tubulin polypeptides themselves. For example, in yeast a significant excess of β-tubulin causes rapid microtubule disassembly and subsequent cell death. Even the modest excess of β-tubulin produced by genetic alterations such as deletion of the minor α-tubulin gene TUB3 affects cell growth and can confer microtubule phenotypes. We show here that the levels of the yeast protein Pac10p affect the relative levels of the tubulin polypeptides. Cells deleted for PAC10 have the same phenotypes as do cells that express reduced levels of α-tubulin or Rbl2p, two proteins that bind β-tubulin. Conversely, overexpression of Pac10p enhances the ability of α-tubulin or Rbl2p to suppress the lethality associated with excess β-tubulin. However, Pac10p is itself not a β-tubulin binding protein. Pac10 null cells show a 30% decrease in the ratio of α-tubulin to β-tubulin. The results suggest that Pac10p modulates the level of α-tubulin in the cell, and so influences microtubule morphogenesis and tubulin metabolism.