Transport of phosphatidylcholine to Xenopus photoreceptor rod outer segments in the presence of tunicamycin.

Study of the dynamics of membrane protein and phospholipid transport from the inner to the outer segment of vertebrate photoreceptors has shown an interesting dissociation of the two components under a number of experimental treatments which inhibit protein synthesis or transport. Under conditions which block the addition of opsin to outer segments, various lipids continue to be synthesized and transported to the outer segment in the presence of monensin, puromycin, brefeldin A, tunicamycin and several general metabolic inhibitors. In the current study, isolated retinas from adult Xenopus laevis were incubated with or without 20 micrograms mg-1 of tunicamycin in total darkness or light for 2-12 h in the presence of [3H]choline to study the dependence of phosphatidylcholine synthesis and transport on protein transport to the outer segment. Phosphatidylcholine is a major bulk lipid of outer segments, comprising close to one half of the phospholipid of outer segment phospholipids, and blocking choline uptake in retinas is known to cause photoreceptor degeneration. Biochemical analysis demonstrates that tunicamycin does not block the synthesis of phosphatidylcholine in photoreceptor inner segments or transport of radiolabelled phosphatidylcholine to outer segments during 6 h incubations with [3H]choline in light or total darkness. Light and electron microscopic autoradiography and morphometric analysis show that [3H]choline radiolabelled phospholipid does not accumulate in a band of newly formed basal discs in the outer segment or in the tubulo-vesicular structures which accumulate in the intersegmental space of tunicamycin-treated retinas. We conclude that transport of phosphatidylcholine can occur independently of opsin transport to the outer segment but whether this represents two separable components of a single pathway or involves two distinct routes of transport to the outer segment is still unresolved.