The post-translational 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. The role of the palmitate residues is unknown but could be related to membrane assembly. The addition of the vitamin A chromophore seem to be essential for intracellular transport of the opsin protein to the Golgi and to the outer segments. The addition of several sugar residues in the Golgi complex may be a requirement for normal outer segment disc formation since the rhodopsin molecules in the plasma membrane and basal folds have a higher molecular weight than rhodopsin in disc membranes. Rod outer segments contain a molecule with both inositol and glucosamine. This molecule is reminiscent of the phosphatidylinositol-glycan anchor found in transiently membrane bound proteins and may indicate the existence of a phospholipase mediated release mechanism. A manganese-dependent 5'-nucleotidase that cleaves cytidine monophosphate has been found to become highly active in rod outer segment tips at the time of disc shedding. It has been isolated, partially purified and characterized and could provide insight into new mechanisms related to the shedding process.
