INTERACTIONS OF NUCLEOTIDE RELEASE FACTOR DSS4P WITH SEC4P IN THE POST-GOLGI SECRETORY PATHWAY OF YEAST

SEC4 is an essential gene encoding a small GTPase that is involved in Golgi to cell surface transport in Saccharomyces cerevisiae and is a p aradigm for studies on the mode of action of Rab proteins. We describe ]here the features of interaction of Sec4p with the accessory protein Dss4p, Dss4p is found both on membranes and in the cytosol; however, i t is the membrane fraction that is complexed to Sec4p. Dss4p, like its mammalian counterpart, Mss4, binds zinc, and disruption of the zinc-b inding site disrupts the ability of the protein to interact with Sec4p , DSS4 overexpression can rescue the lethal phenotype of two alleles o f SEC4, corresponding to dominant mutations of Ras, We demonstrate tha t this suppression is due to the ability of Dss4p to form a tight comp lex with the mutant forms of Sec4p and hence sequester the mutant prot ein from its inhibitory effect. These results imply an in vivo role fo r Dss4p as a guanine nucleotide dissociation stimulator. In vitro the protein has the ability to stimulate the dissociation rate of both GDP and GTP from Sec4p. We examined the relationship of GDI1 and DSS4 wit h SEC4 both genetically and biochemically. These results exclude a rol e for DSS4 in the recruitment of Sec4p/GDI onto membranes.