Phospholipid and pigment alterations after fusion between Rhodobacter sphaeroides chromatophores and acidic liposomes

Wild-type Rhodobacter sphaeroides chromatophores were fused at acidic pH, or by freezing and thawing, with liposomes of soybean phospholipids, phosphatidylserine, phosphatidylglycerol or diphosphatidylglycerol. Equilibrium centrifugation after fusion yielded several fractions. Freeze-fracture electron microscopy showed that fusion resulted in the formation of unilamellar vesicles of diameters larger than that of chromatophores. The lateral density of the intramembrane particles decreased; the asymmetry between the two fracture faces was lost after fusion with soybean phospholipids or phosphatidylserine or phosphatidylglycerol, but gradually disappeared in parallel with diphosphatidylglycerol enrichment. After fusion with phosphatidylserine, when the fractions were frozen from below the lipid transition temperature intramembrane particles aggregated into patches surrounded by smooth lipid zones. A massive incorporation of the fusogen phospholipid was observed in the fractions together with a strong decrease of phosphatidylglycerol and a lower decrease of phosphatidylcholine and aminolipid. The 800 nm absorption band of the B800–850 antenna complex was reduced or suppressed depending on the nature of the lipids while the spectroscopic alteration of B875 chromophore was weaker. The light-induced bandshifts of carotenoid and antenna bacteriochlorophyll were also much weaker or absent; this could result from a desorganization of the B800–850 antenna, or from an impaired capacity to sustain a photoinduced membrane potential. The reaction center was not affected by the fusion, and the polypeptide composition of the various fractions did not show qualitative differences from the chromatophore pattern. Spheroplasts did not show the same capacity of fusion as chromatophores.