The light-harvesting polypeptides of Rhodopseudomonas viridis. The complete amino-acid sequences of B1015-alpha, B1015-beta and B1015-gamma

Abstract

Three low molecular mass polypeptides have been isolated by using the technique of organic solvent extraction of thylakoid membranes or whole cells from Rhodopseudomonas viridis. Their primary structures were determined by long liquid phase sequencer runs, combined with the isolation and sequence analysis of the C-terminal o-iodosobenzoic acid fragment and carboxypeptidase degradation. The polypeptide which consists of 58 amino-acids and is 46% homologous to the antenna polypeptide B880-alpha from Rhodospirillum rubrum was designated as B1015-alpha (1 His residue). The sequence homology between the second polypeptide, named B1015-beta (55 amino acids, 2 His residues) and B880-beta from Rs. rubrum is 52%. For the third polypeptide consisting of 36 amino acids and exhibiting a high hydrophobicity, no equivalent polypeptide has so far been found in other purple bacteria. The molar ratio of these three organic solvent soluble polypeptides from Rp. viridis was estimated to be 1:1:1. Accordingly, the 36 amino-acid polypeptide is likely to be an additional constituent of the light-harvesting complex B1015, consequently termed as B1015-gamma. According to hydrophathy profiles, the transmembrane arrangement of B1015-alpha and B1015-beta within the thylakoid membrane is supposed to be similar. B1015-gamma, however, shows a somewhat different hydropathy profile. A particular feature of this polypeptide is its high amount of aromatic amino acids. It is postulated that B1015-gamma is involved in the formation of regular arrays of light-harvesting complexes.