Cytoplasmic domain of rhodopsin is essential for post-Golgi vesicle formation in a retinal cell-free system

Post-Golgi vesicles cotransport docosahexaenoyl-phospholipids and rhodopsin during frog photoreceptor membrane biogenesis

Post-Golgi vesicles budding from the trans-Golgi network (TGN) are involved in the vectorial transport and delivery of rhodopsin to photoreceptor rod outer segments (ROS). We report here that newly synthesized docosahexaenoyl (DHA) phospholipids are sequestered and cotransported by rhodopsin-bearing post-Golgi vesicles to ROS. Frog retinas were pulse-labeled with [35S]methionine/cysteine and [3H]DHA prior to ROS isolation and subcellular fractionation. After a 1-h pulse, relatively uniform [3H]DHA-lipid labeling (DPM/microg protein) was observed in all fractions enriched in post-Golgi vesicles, TGN, Golgi, and endoplasmic reticulum (ER) membranes. During the subsequent 2-h chase translocation of free [3H]DHA from ROS to the photoreceptor inner segment contributed to an additional overall increase in labeling of lipids. The specific activity (dpm/nmol DHA) in ER-enriched fraction was similar or higher than in other subcellular fractions after both the pulse and the chase, indicating that the bulk of [3H]DHA-lipids was synthesized in the ER. After the chase a 2-fold increase in labeling of lipids in the ER and Golgi and a 2.6-fold in lighter TGN-enriched fractions was observed. The highest labeling was in the post-Golgi vesicle fraction (4-fold increase), with [3H]DHA-phosphatidylcholine and [3H]DHA-phosphatidylethanolamine showing the greatest increase. At the same time, newly synthesized [35S]rhodopsin shifted from the ER and Golgi toward TGN and post-Golgi fractions. Therefore, sequestration and association of [35S]rhodopsin and [3H]DHA-lipids in a TGN membrane domain occurs prior to their exit and subsequent vectorial cotransport on post-Golgi vesicles to ROS. Labeling of ROS lipids was very low, with phosphatidylinositol and diacylglycerols displaying the highest labeling. This indicates that other mechanisms by-passing Golgi, i.e. facilitated by lipid carrier proteins, may also contribute to molecular replacement of disc membrane DHA-phospholipids, particularly phosphatidylinositol.

Site-directed mutagenesis of highly conserved amino acids in the first cytoplasmic loop of Drosophila Rh1 opsin blocks rhodopsin synthesis in the nascent state

The cytoplasmic surface of Drosophila melanogaster Rh1 rhodopsin (ninaE) harbours amino acids which are highly conserved among G-protein-coupled receptors. Site-directed mutations which cause Leu81Gln or Asn86Ile amino acid substitutions in the first cytoplasmic loop of the Rh1 opsin protein, are shown to block rhodopsin synthesis in the nascent, glycosylated state from which the mutant opsin is degraded rapidly. In mutants Leu81Gln and Asn86Ile, only 20-30% and <2% respectively, of functional rhodopsins are synthesized and transported to the photoreceptive membrane. Thus, conserved amino acids in opsin's cytoplasmic surface are a critical factor in the interaction of opsin with proteins of the rhodopsin processing machinery. Photoreceptor cells expressing mutant rhodopsins undergo age-dependent degeneration in a recessive manner.