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 seems to be essential for intracellular transport of the opsin protein to the Golgi and to the outer segments. The addition of galactose residues may be a requirement for normal outer segment disc formation as it appears to be present only in the rhodopsin molecules found in the plasma membrane and basal folds. The polyphosphoinositide pathway has been detected in rat rod outer segments thus extending the known distribution of this pathway from invertebrates and cold-blooded vertebrates to warm-blooded vertebrates. The role of this pathway in either transduction or light adaptation may be universal. 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.
