Immunocytochemical techniques have localized a large protein which is an intrinsic membrane component of isolated frog rod outer segments (ROS). This large protein whose apparent mol wt is 290,000 daltons comprises about 1-3% of the ROS membrane mass.

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Adult vertebrate retinal cells (rod and cones) continuously synthesize membrane proteins and transport them to the organelle specialized for photon capture, the outer segment. The cell structures involved in the synthesis of opsin have been identified by means of immunocytochemistry at the electron microscope level. Two indirect detection systems were used: (a) rabbit antibodies to frog opsin were localized with ferritin conjugated F(ab')2 of sheep antibodies to rabbit F(ab')~ and (b) sheep antibodies to cattle opsin were coupled to biotin and visualized by means of avidin-ferritin conjugates (AvF). The reagents were applied directly to the surface of thin sections of frog retinal tissues embedded in glutaraldehyde cross-linked bovine serum albumin (BSA). Specific binding of anti-opsin antibodies indicates that opsin is localized in the disks of rod outer segments (ROS), as expected, and in the Golgi zone of the rod cell inner segments. In addition, we observed quantitatively different labeling patterns of outer segments of rods and cones with each of the sera employed. These reactions may indicate immunological homology of rod and cone photopigments. Because these quantitative variations of labeling density extend along the entire length of the outer segment, they also serve to identify the cell which has shed its disks into adjacent pigment epithelial cell phagosomes.KEY WORDS rhodopsin membrane along membranous pathways proposed for secrebiosynthesis 9 Golgi zones tion of enzymes. An early hypothesis of Palade immunocytochemistry 9 retina 9 rods andcones (28) modified recently by Hirano et al. (18) postulates the addition of oriented membrane Biosynthesis of cellular membranes may proceed proteins to membranes which move from regions 196 J. CELL BmLOCY 9 The Rockefeller University Press 9

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