Opsin maturation and targeting to rhabdomeral photoreceptor membranes requires the retinal chromophore.

The function of the retinal chromophore in the transcription, translation and targeting of opsin was investigated in fly photoreceptor cells R1-6. Carotenoid deprivation and light-dependent depletion of photoreceptor cells in 11-cis (3-OH) retinal reduced not only the rhodopsin content but also the opsin density of the rhabdomeral membrane by as much as 96%. Electron microscopy revealed that rhabdomeral membranes which lack opsin are morphologically intact, but the cells show a modest proliferation of endoplasmic reticulum and zipper-like differentiations of the plasma membrane adjacent to the microvilli. Opsin mRNA levels, quantified by Northern blot analysis with opsin sense cRNA as an external standard, remain relatively constant (7 x 10(-19) moles opsin mRNA per cell), irrespective of the chromophore-dependent, large changes in the opsin content. Labeling of newly synthesized opsin after injection of eyes with [35S]methionine reveals that opsin mRNA is translated in rhodopsin-depleted cells to the same extent as in rhodopsin-rich cells. Molecular weight changes of metabolically radiolabeled opsin, indicative for the posttranslational processing of the nascent opsin to the mature opsin form, suggest that opsin processing is delayed in chromophore-deprived photoreceptor cells. Time courses of opsin labeling reveal that newly synthesized opsin is degraded faster in chromophore-deprived cells than in cells with a high chromophore supply. These results strongly suggest that the chromophore does not regulate opsin gene transcription, but is required for the processing of opsin and its targeting to the rhabdomeral photoreceptor membranes.