AN ARF1-DELTA SYNTHETIC LETHAL SCREEN IDENTIFIES A NEW CLATHRIN HEAVY-CHAIN CONDITIONAL ALLELE THAT PERTURBS VACUOLAR PROTEIN-TRANSPORT IN SACCHAROMYCES-CEREVISIAE

ADP-ribosylation factor (ARF) is a small GTP-binding protein that is t hought to regulate the assembly of coat proteins on transport vesicles . To identify factors that functionally interact with ARF, we have per formed a genetic screen in Saccharomyces cerevisiae for mutations that exhibit synthetic lethality with an arf1 Delta allele and defined sev en genes by complementation tests (SWA1-7 for synthetically lethal wit h arf1 Delta). Most of the swa mutants exhibit phenotypes comparable t o arf1 Delta mutants such as temperature-conditional growth, hypersens itivity to fluoride ions, and partial protein transport and glycosylat ion defects. Here, we report that swa5-1 is a new temperature-sensitiv e allele of the clathrin heavy chain gene (chc1-5), which carries a fr ameshift mutation near the 3' end of the CHC1 open reading frame. This genetic interaction between arf1 and chc1 provides in vivo evidence f or a role for ARF in clathrin coat assembly. Surprisingly, strains har boring chc1-5 exhibited a significant defect in transport of carboxype ptidase for carboxypeptidase S to the vacuole that was not observed in other chc1 ts mutants. The kinetics of invertase secretion or transpo rt of alkaline phosphatase to the vacuole were not significantly affec ted in the chc1-5 mutant, further implicating clathrin specifically in the Golgi to vacuole transport pathway for carboxypeptidase Y.