The posttranslational modifications of rhodopsin include acylation, glycosylation, and chromophore addition. All appear to take place in the rod inner segment. The resulting molecules exhibit a slightly higher molecular weight than the mature rhodopsin in the outer segment and thus can be distinguished. This higher molecular weight is attributable to the larger oligosaccharide normally found on nascent glycoproteins. Ordinarily, the large oligosaccharide is trimmed and modified as the glycoprotein passes through the Golgi complex. The glycoprotein acquires its mature molecular size by the time it reaches the cell surface. Rhodopsin behaves somewhat differently in that it still exhibits a slightly larger molecular weight after it has been inserted into the plasma membrane of the rod outer segment. Several lines of experimentation have shown that these molecules possess a galactose residue not present in rhodopsin molecules that have been further sequestered in isolated rod outer segment disc membranes. The experimental methods included sensitivity to galactose oxidase and beta-galactosidase as well as affinity chromatography on galactose-specific lectin matrices. As the rhodopsin molecules migrated from the plasma membrane to the disc membranes, they lost labeled galactose and lectin-binding. Separation of plasma membrane and disc membrane fractions confirmed that galactose was trimmed from the oligosaccharide. This unusual sequence of events may be related to disc morphogenesis insofar as tunicamycin blockage of oligosaccharide synthesis results in failure of the rhodopsin-containing membranes to form new discs.
