Isolation and characterization of a subunit form of the B875 light-harvesting complex from Rhodobacter capsulatus

A structural subunit (called B816) has been isolated from the B875 light-harvesting complex of Rhodobacter capsulatus using a detergent-mediated dissociation of chromatophores. Rb. capsulatus MW442 (B800-850- B875+ car+) chromatophores were extracted with benzene and titrated with octyl glucoside (OG) to shift the near-infrared absorption maximum from 873 to 816 nm. Gel filtration chromatography was then used to separate B816 from reaction centers. B816 could be quantitatively shifted back to a B875-like form (lambda max = 875 nm) by decreasing the OG concentration. A similar B816 species could be isolated in low yield from wildtype (B800-850+ B875+ car+) cells but not from SB203E (B800-850- B875+ car-). In the latter case, the B816 subunit seemed too unstable to be isolated under equivalent conditions. The alpha:beta polypeptide ratio, the CD spectrum, and the ability to reversibly dissociate B816 to free bacteriochlorophyll and alpha- and beta-polypeptides were found to be similar to those of the B820 subunit of Rs. rubrum previously reported by our laboratory.