Incorporation of squalene into rod outer segments

We have reported previously that squalene is the major radiolabeled nonsaponifiable lipid product derived from 3H]acetate in short term incubations of frog retinas (Keller, R. K., Fliesler, S. J., and Nellis, S. W. (1988) J. Biol. Chem. 263, 2250-2254). In the present study, we demonstrate that newly synthesized squalene is incorporated into rod outer segments under similar in vitro conditions. We show further that squalene is an endogenous constituent of frog rod outer segment membranes; its concentration is approximately 9.5 nmol/mumol of phospholipid or about 9% of the level of cholesterol. Pulse-chase experiments with radiolabeled precursors revealed no metabolism of outer segment squalene to sterols in up to 20 h of chase. Taken together with our previous absolute rate studies (Keller, R. K., Fliesler, S. J., and Nellis, S. W. (1988) J. Biol. Chem. 263, 2250-2254), these results suggest that most, if not all, of the squalene synthesized by the frog retina is transported to rod outer segments. Synthesis of protein is not required for squalene transport since puromycin had no effect on squalene incorporation into outer segments. Conversely, inhibition of isoprenoid synthesis with mevinolin had no effect on the incorporation of opsin into the outer segment. These latter results support the conclusion that the de novo synthesis and subsequent intracellular trafficking of opsin and isoprenoid lipids destined for the outer segment occur via independent mechanisms.