Pulse–chase labeling of the M2, M3, and M5 mutants. COS-1 cells transiently transfected with mutant opsins were labeled for 5 min with [35S]methionine and [35S]cysteine and incubated in unlabeled methionine and cysteine for indicated chase time in minutes above each lane. Opsin was immunoprecipitated with the 1D4 antibody, resolved by SDS/PAGE (10% for M5; 12.5% for M2, and M3), and visualized by fluorography.