TUNICAMYCIN DOES NOT INHIBIT TRANSPORT OF PHOSPHATIDYLINOSITOL TO XENOPUS ROD OUTER SEGMENTS

Tunicamycin inhibits the dolichol pathway for N-linked glycosylation o f proteins, including photoreceptor opsin, and causes a buildup of tub ulo-vesicular profiles in the intersegmental space between photorecept or rod inner and outer segments associated with disruption of new disc assembly. We tested the hypothesis that a tunicamycin lesion in photo receptors would block lipid transport into the outer segment. Adult Xe nopus retinas were preincubated in dim red light with 20 mug ml-1 of t unicamycin for one hour followed by incubation in the light for 2-6 h with tunicamycin plus either [H-3]mannose, [H-3]leucine, [2-H-3]glycer ol or [H-3]myo-inositol. Tunicamycin caused accumulation of tubulo-ves icular membranes in the intersegmental space and significantly reduced both [H-3]leucine and [H-3]mannose incorporation into the basal regio n of rod outer segments. However, tunicamycin had no effect on [H-3]gl ycerol incorporation into the rod outer segment phospholipids. After 5 h incubation with [H-3]glycerol, radiolabel in outer segment fraction s was associated primarily with phosphatidylinositol in both control a nd tunicamycin treated retinas. Quantitative light microscope autoradi ography of both [H-3]glycerol and [H-3]inositol labelled retinas showe d diffuse labelling over the entire rod outer segment in both control and tunicamycin treated retinas with no accumulation of radioactivity in the basal discs of control retinas or in the tubulo-vesicular struc tures in the intersegmental space of tunicamycin treated retinas. Our results indicate that despite the morphological disruption and inhibit ion of glycoprotein transport to outer segments after tunicamycin trea tment, transport of labelled phosphatidylinositol occurs normally. The se data add to a growing body of evidence separating the lipid and pro tein transport pathways to the outer segment.