Characterization of the SAC3 gene of Saccharomyces cerevisiae

A temperature-sensitive mutation (act1-1) in the essential actin gene of Saccharomyces cerevisiae can be suppressed by mutations in the SAC3 gene. A DNA fragment containing the SAC3 gene was sequenced. SAC3 codes for a 150 kDa hydrophillic protein which does not show any significant similarities with other proteins in the databases. Sac3 therefore is a novel yeast protein. A nuclear localization of Sac3 is suggested by the presence of a putative nuclear localization signal in the Sac3 sequence. A SAC3 disruption mutation was constructed. SAC3 disruption mutants were viable but grew more slowly and were larger than wild-type cells. In contrast to the sac3-1 mutation, the SAC3 disruption was not able to suppress the temperature sensitivity and the osmosensitivity of the act1-1 mutant. This demonstrates that act1-1 suppression by sac3-1 is not the result of a simple loss of SAC3 function. Furthermore, we examined the act1-1 and the sac3 mutants for defects in polarized cell growth by FITC-Concanavalin A (Con A)-labelling. The sac3 mutants showed a normal ConA-labelling pattern. In the act1-1 mutant, however, upon shift to non-permissive temperature, newly synthesized cell wall material, instead of being directed towards the bud, was deposited at discrete spots in the mother cell.