Addition of the chromophore to rat rhodopsin is an early post-translational event

Rat retinas were labeled either by intravitreal injection of [¹⁴C]leucine or by incubation with [³H]-leucine or [³⁵S]-methionine. Subcellular fractions were prepared on linear sucrose gradients and rhodopsin was extracted with detergent and purified by chromatogaphy on ConA-Sepharose. A fraction enriched in rough endoplasmic reticulum (RER) and substantially free of rod outer segments (ROS) was found to contain a light-sensitive protein exhibiting the properties of rhodopsin on ConA-Sepharose or Agarose chromatography and on SDS-polyacrylamide gel electrophoresis, as well as immunologically. Intravitreal injection of [³H]-retinol also labeled the rhodopsin in the RER under conditions in which the rhodopsin in the ROS was not heavily labeled. Thus the chromophore appears to be attached to opsin shortly after the apoprotein is translated in the RER.