Antibody binding sites were localized by using synthetic C-terminal peptides in conjunction with solid-phase competitive inhibition assays and limited proteolytic digestion of rhodopsin in conjunction with electrophoretic immunoblotting techniques. Binding of the rho 1D4 and rho 3C2 antibodies to immobilized rhodopsin was inhibited with peptides of length 1-8'and longer. Antibody rho1D4 binding was not inhibited by peptides 2'-13'or 3'-18', indicating that the C-terminal alanine residue of rhodopsin was required. Similar competitive inhibition studies indicated that the an-tibody rho 3A6 required peptides of length 1'—12'and longer whereas rho 1C5 required peptide 1'—18'. Peptide 3'—18'was as effective as 1'—18'in inhibiting rho 3A6 binding to rho-dopsin, but replacement of glutamic acid in position 8'with glutamine abolished competition. This substitution had little effect on the binding of antibody rho 1C5. Thus, Glu8'was essential for rho 3A6 binding butnot for the binding of the rho 1C5 antibody. Cleavage of the seven amino acid C-ter-minus from rhodopsin and further cleavage to F [[M, 25 000) and F2 (Mt 12000) fragments with Staphylococcus aureus

! R. hodopsin is the photoreceptor protein of vertebrate rod cells. It consists of a polypeptide chain of Afr 39 000 linked to a molecule of 11-cw-retinal [see Hargrave (1982) for a recent review]. The complete covalent sequence of the protein has recently been determined (Ovinchinnikov et al., 1982; Hargrave et al., 1983). Topographic studies carried out in a number of laboratories have shown that the carboxyl-terminal segment of rhodopsin is exposed on the cytoplasmic side of the disk membrane and theamino-terminal segment con-