Membranes of Rhodospirillum rubrum: physicochemical properties of chromatophore fractions isolated from osmotically and mechanically disrupted cells

Abstract

Isolation of highly purified membrane fractions from phototropically grown R. rubrum was achieved by velocity and isopyknic sedimentation under carefully controlled ionic conditions. Bacteriochlorophyll-rich and succinic dehydrogenase-rich chromatophores that were essentially devoid of contamination by non-chromatophore protein were separated from a denser fraction in extracts disrupted in a French pressure cell. Highly purified chromatophores and a nearly photopigment-free envelope fraction were also obtained from cells lysed by treatment with EDTA-lysozyme-Brij 58. After lysis with lysozyme and EDTA alone, about 50% of the total photosynthetic pigment was released in chromatophores similar to those isolated by the above procedures. Chromatophores prepared by each method had very similar near absorption spectra, overall chemical composition, equilibrium buoyant densities in CsCl and protein patterns in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein profiles of the dense, outer membrane-rich fractions were different from those of the chromatophores. The release of much of the photosynthetic apparatus as discrete chromatophores in osmotically lysed extracts necessitates a reevaluation of the concept that isolated chromatophores arise only from mechanical comminution of a larger membrane structure.