TRANSPORT OF PHOSPHATIDYLCHOLINE TO XENOPUS PHOTORECEPTOR ROD OUTER SEGMENTS IN THE PRESENCE OF TUNICAMYCIN

Study of the dynamics of membrane protein and phospholipid transport f rom the inner to the outer segment of vertebrate photoreceptors has sh own an interesting dissociation of the two components under a number o f 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 out er segment in the presence of monensin, puromycin, brefeldin A, tunica mycin and several general metabolic inhibitors. In the current study, isolated retinas from adult Xenopus laevis,is were incubated with or w ithout 20 mu g mg(-1) of tunicamycin in total darkness or light for 2- 12 h in the presence of [H-3]choline to study the dependence of phosph atidylcholine synthesis and transport on protein transport to the oute r segment. Phosphatidylcholine is a major bulk lipid of outer segments , comprising close to one half of the phospholipid of outer segment ph ospholipids, and blocking choline uptake in retinas is known to cause photoreceptor degeneration. Biochemical analysis demonstrates that tun icamycin does not block the synthesis of phosphatidylcholine in photor eceptor inner segments or transport of radiolabelled phosphatidylcholi ne to outer segments during 6h incubations with [H-3]choline in light or total darkness. Light and electron microscopic autoradiography and morphometric analysis show that [H-3]choline radiolabelled phospholipi d does not accumulate in a band of newly formed basal discs in the out er segment or in the tubulo-vesicular structures which accumulate in t he intersegmental space of tunicamycin-treated retinas. We conclude th at transport of phosphatidylcholine can occur independently of opsin t ransport to the outer segment but whether this represents two separabl e components of a single pathway or involves two distinct routes of tr ansport to the ou ter segment is still unresolved.