The primary structures of the core antenna polypeptides from Rhodopseudomonas marina

Abstract

The antenna complex B880 of Rp. marina has been isolated by applying ion-exchange chromatography on Whatman DE-52 resin and sucrose density centrifugation of LDAO-solubilized photosynthetic membranes. The antenna polypeptides B880-alpha and B880-beta were prepared by organic solvent extraction of extensively dialyzed and freeze-dried B880 antenna complex material or photosynthetic membranes. Gel filtration on Sephadex LH-60 and ion-exchange chromatography on Whatman DE-32 resin in the presence of organic solvents and an additional step on a C-8 reversed phase column yielded pure alpha- and beta-apoproteins. Their complete primary structures have been elucidated using automated Edman degradation and carboxypeptidase digestion. According to quantitative Edman degradation the ratio of B880-alpha and B880-beta has been determined as 1:1 in the isolated antenna complex as well as in the photosynthetic membrane. B880-alpha of Rp. marina, presumably N-formylated, consists of 52 amino acid residues and is 75, 56, 52 and 44% homologous to the corresponding core antenna polypeptides of Rs. rubrum, Rp. viridis, Rb. capsulatus and Rb. sphaeroides. In contrast, B880-beta (56 amino acid residues) is less homologous to the corresponding core beta-antenna polypeptides of the same strains (57, 51, 41 and 42%). It shows an extended N-terminal domain as compared to the B880-alpha polypeptide. Apart from the typical structural features of bacterial membrane-bound antenna polypeptides (three domain structure. His-residue in the hydrophobic stretch) the antenna polypeptides of Rp. marina are structurally related to polypeptides of core antenna complexes with strong near infrared circular dichroism signals.