BIOSYNTHESIS AND ASSEMBLY OF ROD OUTER SEGMENT MEMBRANE SYSTEM . FORMATION AND FATE OF VISUAL PIGMENT IN FROG RETINA

When a mixture of tritiated leucine and phenylalanine is injected into adult frogs, the amino acids are rapidly incorporated into newly synthesized protein in the retinal rod inner segment. Electron microscope autoradiography shows that these radioactive proteins are assembled into the basal membranous discs of the outer segment following a two-hour period of migration within the inner segment. This assembly into disc structure is coincident with a sudden rise in the specific activity of visual pigment500 (rhodopsin) extracted from isolated, washed outer segments and purified by gel filtration. 80 to 85% of the radioactivity in washed outer segments is recovered in the visual pigment fraction. Maximum labeling of visual pigment occurs by six hours post injection, after which the specific activity remains constant for the next 8.5 weeks. During this period, autoradiograms show a successive displacement of labeled discs towards the apex of the outer segment. During this time, disc labeling neither increases nor decreases in its intensity. These findings indicate that visual pigment apoprotein does not undergo turnover during the period that it forms a structural part of the membranous disc. After 8.5 weeks there is a gradual drop in the specific activity of visual pigment. At the same time, the labeled discs disappear from the apex of the outer segment and appear as membranous inclusions undergoing degradation in the cytoplasm of the pigment epithelium. These experiments show that one structural component of the rod outer segment disc membranes, the visual pigment apoprotein, is constantly renewed. Once assembled into the disc structure, the apoprotein of visual pigment remains stable until it is destroyed by the pigment epithelium. On the basis of the experimental results, a model for the synthesis and assembly of rod outer segment disc membranes is proposed. © 1969.