A SINGLE BINDING-SITE FOR DILYSINE RETRIEVAL MOTIFS AND P23 WITHIN THE GAMMA-SUBUNIT OF COATOMER

Coatomer, the major component of the coat of COPI transport vesicles, binds both to the dilysine motif of resident membrane proteins of the endoplasmic reticulum and to the cytoplasmic domain of p23, a major ty pe I membrane protein of COPI vesicles. Using a photocrosslinking appr oach, we find that under native conditions a peptide analogous to the cytoplasmic domain of p23 interacts with coatomer exclusively through its gamma subunit and shares its binding site with a KKXX retrieval mo tif, However, upon dissociation of coatomer, interaction with various subunits, including an alpha-, beta'-, epsilon-COP subcomplex, of the photoreactive peptide is observed. We suggest that, under physiologica l conditions, interaction of coatomer with both endoplasmic reticulum retrieval motifs and the cytoplasmic domain of p23 is mediated by gamm a-COP.