Ras oncogene proteins are plasma membrane-associated signal transducers that are found in all eukaryotes. Posttranslational addition of lipid to a carboxyl-terminal CaaX box (where "C" represents a cysteine, "a" is generally an aliphatic residue, and X can be any amino acid) is required to target Ras proteins to the cytosolic surface of the plasma membrane. The pathway by which Ras translocates from the endoplasmic reticulum to the plasma membrane is currently not clear. We have performed a genetic screen to identify components of the Ras plasma membrane localization pathway. Mutations in two genes, ERF2 and ERF4/SHR5, have been shown to affect the palmitoylation and subcellular localization of Ras proteins. In this report, we show that Erf4p is localized on the endoplasmic reticulum as a peripheral membrane protein in a complex with Erf2p, an integral membrane protein that was identified from the same genetic screen. Erf2p has been shown to be required for the plasma membrane localization of GFP-Ras2p via a pathway distinct from the classical secretory pathway (X. Dong and R. J. Deschenes, manuscript in preparation). We show here that Erf4p, like Erf2p, is involved in the plasma membrane localization of Ras2p. Erf2p and Erf4p represent components of a previously uncharacterized subcellular transport pathway involved in the plasma membrane targeting of Ras proteins.