THE SAG3 GENE ENCODES A NUCLEAR-PROTEIN REQUIRED FOR NORMAL PROGRESSION OF MITOSIS

The SAC3 gene of Saccharomyces cerevisiae has been implicated in actin function by genetic experiments showing that a temperature sensitive mutation in the essential actin gene (act1-1) can be suppressed by mut ations in SAC3. An involvement of SAC3 in actin function is further su ggested by the observation that the actin cytoskeleton is altered in S AC3 mutants. Our fractionation experiments, however, point to a nuclea r localization of Sac3p. On sucrose density gradients Sac3p co-fractio nated with the nuclear proteins Ssb1p, Nop1p and Nop2p but not with th e other organelle markers examined. Furthermore, Sac3p was enriched 10 -fold in a nuclei preparation along with the nuclear protein Nop1p. In this report we further show that SAC3 function is required for normal progression of mitosis. SAC3 mutants showed a higher fraction of larg e-budded cells in culture, indicative of a cell cycle delay. The predo minant population among the large-budded sac3 cells were cells with a single nucleus at the bud-neck and a short intranuclear spindle. This suggests that a cell cycle delay occurs in mitosis prior to anaphase. The observation that SAC3 mutants lose chromosomes with higher frequen cy than wild-type is another indication for a mitotic defect in SAC3 m utants. We further noticed that SAC3 mutants are more resistant agains t the microtubule destabilizing drug benomyl. This finding suggests th at SAC3 is involved, directly or indirectly, in microtubule function. In summary, our data indicate that SAC3 is involved in a process which affects both the actin cytoskeleton and mitosis.