Coatomer, a cytosolic heterooligomeric protein complex that consists of seven subunits [alpha-, beta-, beta'-, gamma-, delta-, epsilon-, and zeta-COP (nonclathrin coat protein)], has been shown to interact with dilysine motifs typically found in the cytoplasmic domains of various endoplasmic-reticulum-resident membrane proteins [Cosson, P. & Letourneur, F. (1994) Science 263, 1629-1631]. We have used a photo-cross-linking approach to identify the site of coatomer that is involved in binding to the dilysine motifs. An octapeptide corresponding to the C-terminal tail of Wbp1p, a component of the yeast N-oligosaccharyltransferase complex, has been synthesized with a photoreactive phenylalanine at position -5 and was radioactively labeled with [125I]iodine at a tyrosine residue introduced at the N terminus of the peptide. Photolysis of isolated coatomer in the presence of this peptide and immunoprecipitation of coatomer from photo-cross-linked cell lysates reveal that gamma-COP is the predominantly labeled protein. From these results, we conclude that coatomer is able to bind to the cytoplasmic dilysine motifs of membrane proteins of the early secretory pathway via its gamma-COP subunit, whose complete cDNA-derived amino acid sequence is also presented.