Structural studies on reconstituted reaction center-phosphatidylcholine membranes

Abstract

Reaction center protein, isolated from the photosynthetic bacterium Rhodopseudomonas sphaeroides R26 mutant, was incorporated into phosphatidylcholine bilayers forming a homogeneous population of unilamellar vesicles. Cytochrome c, added to preformed reaction center-phosphatidylcholine vesicles, rapidly reduced up to 90% of the laser-generated (BChl)2+ of the reaction center (with kinetics of electron transfer similar to those in the chromatophore membrane) which suggests that the portion of the reaction center which accommodates functional cytochrome c binding sites is exposed predominantly on the exterior of the vesicles. Unit cell electron density profiles were derived from lamellar X-ray diffraction from oriented reaction center-phosphatidylcholine membrane multilayers at varying lipid/protein ratios. The analysis of these profiles showed that the reaction center protein incorporates into the phosphatidylcholine membrane with unique sidedness and that the profile of the reaction center protein itself is asymmetric and spans the membrane.