NONCLATHRIN COAT PROTEIN-GAMMA, A SUBUNIT OF COATOMER, BINDS TO THE CYTOPLASMIC DILYSINE MOTIF OF MEMBRANE-PROTEINS OF THE EARLY SECRETORY PATHWAY

Coatomer, a cytosolic heterooligomeric protein complex that consists o f 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. & Letour neur, F. (1994) Science 263, 1629-1631]. We have used a photo-cross-li nking approach to identify the site of coatomer that is involved in bi nding to the dilysine motifs, An octapeptide corresponding to the C-te rminal tail of Wbp1p, a component of the yeast N-oligosaccharyltransfe rase complex, has been synthesized with a photoreactive phenylalanine at position -5 and was radioactively labeled with [I-125]iodine at a t yrosine residue introduced at the N terminus of the peptide, Photolysi s of isolated coatomer in the presence of this peptide and immunopreci pitation 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 mo tifs of membrane proteins of the early secretory pathway via its gamma -COP subunit, whose complete cDNA-derived amino acid sequence is also presented.