Isolation and characterization of an organic solvent soluble polypeptide component from photoreceptor complexes of Rhodospirillum rubrum

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Abstract
An organic solvent soluble polypeptide was isolated from photoreceptor complexes and chromatophores of R. rubum. After extraction of the protein from lyophilized samples with 1:1 chloroform-methanol, it was purified by column chromatography. Its isoelectric point determined by isoelectric focusing was 7.10. When analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified polypeptide ran as a single band of an apparent MW of 12,000. However, according to amino acid analysis, the minimal MW based on 1 histidine residue/polypeptide is 19,000. The polypeptide contains no cysteine or tyrosine. Amino acid analyses indicated that 3 methionines were present/histidine residue, and CNBr bromide cleavage gave 4 smaller peptides which were isolated by 2-dimensional electrophoresis and chromatography. Spectroscopic analysis indicated the presence of 3 tryptophan residues/histidine, and N-bromosuccinamide cleavage gave 4 smaller peptides which could be isolated by 2-dimensional electrophoresis and chromatography. The C[carboxy]-terminal amino acid was glycine by 2 methods, while the N[amino]-terminal amino acid appears to be blocked. The organic solvent soluble polypeptide accounts for .apprx. 50% of the chromatophore protein and seems to bind the antenna bacteriochlorophyll and carotenoid molecules. Using this procedure, organic solvent soluble polypeptides were isolated from several photosynthetic bacteria and had substantially different amino acid contents.