For membrane protein-enriched experiments, four retinas were dissected, homogenized, and briefly sonicated in 1 mL sample buffer (0.1 M NaCl and 50 mM Na2HPO4, pH 7.2) with proteinase inhibitors (Roche Diagnostic Corporation, Indianapolis, IN). The insoluble fraction was collected after centrifugation at 15,000 rpm for 15 minutes at 4°C. The pellet was washed with fresh sample buffer once. One hundred microliters of NP-40 lysis buffer (1% NP-40, 150 mM NaCl, 50 mM Tris, pH 8.0, and one tablet of proteinase inhibitor per 10 mL solution) was added to the pellet and sonicated. After centrifugation at 15,000 rpm for 15 minutes at 4°C, the supernatant was collected, and the protein concentration was determined (Coomassie Assay Reagent kit; Pierce, Rockford, IL). The samples were then mixed with an equal volume of 1× loading buffer (60 mM Tris-HCl, pH 6.8, 2% SDS, 10 mM dithiothreitol, 10% glycerol, and 0.0001% bromophenol blue). Equal amounts of the samples were loaded onto 12.5% PAGE for separation and subjected to Western blot analysis. Rhodopsin proteins were detected with a mouse anti-rhodopsin monoclonal antibody (Chemicon, Temecula, CA), and β-actin was detected with mouse monoclonal β-actin antibody (Sigma, St. Louis, MO).