Evidence for the transport of opsin in the connecting cilium and basal rod outer segment in rat retina: rapid-freeze, deep-etch and horseradish peroxidase labelling studies

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Abstract
Summary In order to clarify the pathway of opsin transport in the connecting cilium and basal rod outer segment, we examined rat rod cells by a rapid-freeze and deep-etch technique and also examined the uptake of horseradish peroxidase into isolated retina. The distribution of intramembrane particles on the P-face of the cilium indicated that the ciliary plasma membrane has similar opsin content to the basal rod outer segment plasma membrane. Dilated cisternae were detected below the stack of disk membranes at the basal rod outer segment in fresh retina. The fine structure of the P-face and true surface of these cisternae was identical to that of the disk membrane. Uptake of horseradish peroxidase was detected in the cisternae or in both cisternae and most basal disk, indicating that the cisternae are formed prior to the disk membrane. In the distal part of connecting cilium, we found axially oriented infoldings on the P-face of the plasma membrane, and subplasmalemmal tubules or cisternae adjacent and parallel to them. Such subplasmalemmal membranes were labeled by exogeneous horseradish peroxidase, suggesting that the infoldings are invaginating plasma membrane. These results may indicate that opsin molecules are conveyed on the ciliary plasma membrane, and that this opsin-rich plasma membrane is internalized in the distal connecting cilium to form dilated cisternae, which subsequently change to the disk membranes.