The N-terminal residue of bovine rhodopsin is acetylmethionine.

By affinity chromatography on a concanavalin A-Sepharose column and gel filtration on a Sephadex G-25 column, a glycopeptide of 16 amino acid residues has been separated from a tryptic digest of reduced and carboxymethylated bovine rhodopsin. The glycopeptide was treated with cyanogen bromide and products were subjected to high-voltage paper electrophoresis. N-Blocked homoserine separated was reacted first with anhydrous hydrazine and then with dansyl chloride. The product was identified by thin-layer chromatography to be 1-acetyl-2-dansyl hydrazine, thus showing that the N-terminal blocking group was an acetyl group. The remaining peptide after cyanogen bromide treatment was partially sequenced by the Edman dansylation method. The present results and previously reported findings on the binding site of the sugar moiety, taken together, indicate that the N-terminal heptapeptide has the following structure: acetylMet-Asn(carbohydrate)-Gly-Thr-Glx-Gly-Pro.