Characterization of the three major intracytoplasmic membrane polypeptides isolated from Rhodopseudomonas sphaeroides

The characterization of the three major membrane polypeptides indicated to be subunits of the light-harvesting bacteriochlorophyll-protein complexes of Rhodopseudomonas sphaeroides is described. Molecular weight determinations of the three polypeptides have been performed by sedimentation analysis and electrophoretic mobility and the values obtained were in agreement with each other as well as with those derived from compositional studies. NH2- and COOH-terminal amino acid determinations were performed for each of the species; two of the polypeptides produces no reaction with dansyl chloride and were concluded to have blocked amino termini. In each instance, the data were consistent with each of the polypeptides being a single, pure species. Further characterization of the proteins was accomplished by amino acid compositional analysis and peptide mapping of tryptic and chymotryptic digestions of the polypeptides. The number of peptides obtained from the digestions was in agreement with the amino acid compositional data. Two of the polypeptides, 15B and 15C, which possess very similar molecular weights, identical carboxyl termini and apparently blocked amino termini were shown by amino acid analysis and peptide mapping to be structurally discrete species, although related. The relationship between these purified proteins and the known activities and characteristics of the light-harvesting bacteriochlorophyll-protein complexes of R. sphaeroides is discussed.