Localisation of the subunits of the photosynthetic reaction centers in the chromatophore membrane of Rhodospirillum rubrum.

Reaction centers were isolated with the detergent lauryl dimethyl amine oxide from chromatophore membranes of Rhodospirillum rubrum. The subunit composition of these reaction centers is similar to the one obtained from Rhodopseudomonas spheroides: three subunits with the molecular weights of 21 000, 24 000 and 29 000. Reaction centers prepared from chromatophores labeled with 131I were heavely labeled in their large subunit (H). The smaller subunits (L and M) contained only little label. Sonication during labeling yielded a slightly higher incorporation of 131I in subunit H compared to the smaller ones. It is concluded that the H protein is largely exposed at the cytoplasmic side of the membrane but might also be accessible for iodination on the inside of the membrane while the L and M proteins are almost completely embedded in the membrane. Iodination of spheroplasts results in only a slight binding of 131I to chromatophores and reaction centers.