Association of LHIalpha(B870) polypeptide with phospholipids during insertion in the photosynthetic membrane of an LHII- mutant of Rhodobacter capsulatus

Membranes from in vivo labeled cells of Rhodobacter capsulatus U43[pTX35] grown photosynthetically carried 60% of the [32P]-Pi in the "heavy" fraction (HM) after sucrose gradient sedimentation. Metal-chelating chromatography of either"heavy" or "light" (LM) membrane fractions rendered similar Bchl-protein complex profiles after octyl-glucoside treatment,including most of the radioactivity in the same corresponding elution fraction (F II). Similar labeling distribution of pigment-protein complexes was obtained for membranes of dark-grown cells induced by lowering oxygen tension. Fractions derived from HM showed highly labeled LHIalpha, whereas the same complex from LM was essentially [32P]-Pi-free, as revealed by SDS-PAGE followed by autoradiography. Phospholipid analysis showed a similar pattern for membranes isolated from cells photosynthetically or semiaerobically grown, being the most abundant: phosphatidylglycerol,phosphatidylethanolamine, cardiolipin, and phosphatidylcholine. Part of the phospholipids from HM comigrated with LHIalpha during SDS-PAGE and dissociated from the complexes only after solvent extraction and hydrophobic chromatography. However, a small amount remained always attached to LHIalpha,indicating an unusual strong interaction. These results suggest the existence of two operationally defined membrane regions carrying LHIalpha complexes differing in phosphorylation status and protein-phospholipid interaction.