Membrane topography of the photosynthetic reaction center polypeptides of Rhodopseudomonas sphaeroides

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Abstract
The topography of the photosynthetic reaction center (RC) polypeptides (H, M and L) was investigated by proteolysis and radioiodination of membrane vesicles isolated from R. sphaeroides. Chromatophores, obtained from French-pressed cell lysates, are closed vesicles and oriented inside out with respect to the cytoplasmic membrane (cytoplasmic side out). Spheroplast-derived vesicles (SDV), obtained after osmotic lysis of lysozyme-treated cells, are oriented right side in (periplasmic side out). .alpha.-Chymotrypsin treatment of chromatophores and trypsin treatment of SDV resulted in cleavage of H. .alpha.-Chymotrypsin treatment of SDV did not cleave H and trypsin treatment of chromatophores did not consistently cleave this polypeptide. M and L of both vesicles were apparently not affected by these proteases. The SDV trypsin cleavage product of H was identified by .alpha.-chymotryptic 125I-labeled peptide mapping and had a MW of 26,000. Membrane surface radioiodination with chloroglycoluril coated on glass tubes resulted in preferential labeling of H and M of SDV and chromatophores. The radiospecific activities of H, M and L were higher with labeling of SDV as compared to labeling of chromatophores. .alpha.-Chymotryptic 125I-labeled peptide maps of H, M and L from surface-radioiodinated SDV differed from the corresponding maps of these polypeptides from surface-radioiodinated chromatophores. H, M and L are apparently asymmetrically exposed on opposite surfaces of the R. sphaeroides membrane. Exposed iodination sites of these polypeptides are more abundant on the periplasmic surface than on the cytoplasmic surface of this membrane.

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