The primary structure of a light-harvesting bacteriochlorophylla-binding protein of wild-type Rhodospirillum rubrum

The complete amino acid sequence has been obtained for a major light-harvesting polypeptide of wild-type Rhodospirillum rubrum. The peptide was isolated from lyophilized chromatophores by extraction with chloroform/methanol (1:1, v:v) and then purified by gel filtration chromatography using the same solvent with 0.1 M ammonium acetate added. The methods used for sequence determination were: (1) partial acid hydrolysis followed by separation of small peptides on a Beckman PA-35 column and then determination of their amino acid sequences by manual Edman degradation, (2) unblocking the N-terminus by mild acid treatment and subjecting the resulting polypeptide to automated Edman degradation, (3) cleavage by o-iodosobenzoic acid treatment, isolation of the C-terminal fragment by chromatography on a Bio-Rad P-2 column and sequencing by automated Edman degradation, and (4) determination of the C-terminus by carboxypeptidase Y treatment. The amino acid sequence obtained was: Nf-Met-Trp-Arg-Ile-Trp-Gln-Leu-Phe-Asp-Pro-Arg-Gln-Ala-Leu-Val-Gly-Leu-Ala-Thr- Phe-Leu-Phe-Val-Leu-Ala-Leu-Leu-Ile-His-Phe-Ile-Leu-Leu-Ser-Thr-Glu-Arg-Phe-Asn-Trp-Leu-Glu-Gly-Ala-Ser-Thr-Lys-Pro-Val-Gln-Thr-Ser-COOH. This is identical to the previously determined amino acid sequence of a light-harvesting polypeptide from the G-9 carotenoid-less mutant of R. rubrum. The long stretch of consecutive apolar amino acids suggests this polypeptide may span the membrane with an α-helical segment.