Accessibility of the carbohydrate moiety of membrane-bound rhodopsin to enzymatic and chemical modification

Galactose was specifically inserted into the carbohydrate moiety of rhodopsin by incubating retinal disk membranes with UDP-galactose: N-acetylglucosamine galactosyltransferase. The stoichiometry of labeling ranged from 1.2 to 1.8 (average = 1.5) residues of galactose per molecule of rhodopsin, indicating that some or all of the oligosaccharide chains of membrane-bound rhodopsin are readily accessible to enzymatic modification. These modified membranes were treated with galactose oxidase to generate an aldehyde at the C-6 position of the inserted galactose units. The enzymatically-oxidized membranes were then reacted with dansyl hydrazide to yield a fluorescent hydrazone which is sufficiently stable to permit spectroscopic analysis. This procedure for the specific attachment of a spectroscopic probe should be applicable to a wide variety of membrane glycoproteins.