A NUCLEOLAR PROTEIN THAT AFFECTS MATING EFFICIENCY IN SACCHAROMYCES-CEREVISIAE BY ALTERING THE MORPHOLOGICAL RESPONSE TO PHEROMONE

SSF1 and SSF2 are redundant essential yeast genes that, when overexpre ssed, increase the mating efficiency of cells containing a defective S te4p G(beta) subunit. To identify the precise function of these genes in mating, different responses to pheromone were assayed in cells that either lacked or overexpressed SSF gene products. Cells containing nu ll alleles of both SSF1 and SSF2 displayed the normal transcriptional induction response to pheromone but were unable to form mating project ions. Overexpression of SSF1 conferred the ability to form mating proj ections on cells containing a temperatures-sensitive STE4 allele, but had only a small effect on transcriptional induction. SSF1 overexpress ion preferentially increased the mating efficiency of a strain contain ing a null allele of SPA2, a gene that functions specifically in cell morphology. To investigate whether Ssf1p plays a direct physical role in mating projection formation, its subcellular location was determine d. An Ssf1p-GFP fusion was found to localize to the nucleolus, implyin g that the role of SSF gene products in projection formation is indire ct. The region of Ssf1p-GFP localization in cells undergoing projectio n formation was larger and more diffuse, and was often present in a sp ecific orientation with respect to the projection. Although the functi on of Ssf1p appears to originate in the nucleus, it is likely that it ultimately acts on one or more of the proteins that is directly involv ed in the morphological response to pheromone. Because many of the pro teins required for projection formation during mating are also require d for bud formation during vegetative growth, regulation of the activi ty or amount of one or more of these proteins by Ssf1p could explain i ts role in both mating and dividing cells.