14-3-3-PROTEINS - POTENTIAL ROLES IN VESICULAR TRANSPORT AND RAS SIGNALING IN SACCHAROMYCES-CEREVISIAE

Deletion of the clathrin heavy-chain gene, CHC1, in the budding yeast Saccharomyces cerevisiae results in growth, morphological, and membrane trafficking defects, and in some strains chc1-Delta is lethal. A previous study identified five genes which, in multicopy, rescue inviable strains of Chc(-) yeast. Now we report that one of the suppressor loci, BMH2/SCD3, encodes a protein of the 14-3-3 family. The 14-3-3 proteins are abundant acidic proteins of approximate to 30 kDa with numerous isoforms and a diverse array of reported functions. The Bmh2 protein is >70% identical to the mammalian epsilon-isoform and >90% identical to a previously reported yeast 14-3-3 protein encoded by BMH1. Single deletions of BMH1 or BMH2 have no discernible phenotypes, but deletion of both BMH1 and BMH2 is lethal. High-copy BMH1 also rescues inviable strains of Chc(-) yeast, although not as well as BMH2. In addition, the slow growth of viable strains of Chc(-) yeast is further impaired when combined with single bmh mutations, often resulting in lethality. Overexpression of BMH genes also partially suppresses the temperature sensitivity of the cdc25-1 mutant, and high-copy TPK1, encoding a cAMP-dependent protein kinase, restores Bmh(-) yeast to viability. High-copy TPK1 did not rescue Chc(-) yeast. These genetic interactions suggest that budding-yeast 14-3-3 proteins are multifunctional and may play a role in both vesicular transport and Ras signaling pathways.