Possible role of the highly conserved amino acids Trp‐8 and Pro‐13 in the N‐terminal segment of the pigment‐binding poly eptide LHI α of Rhodobacter capsulatus

Abstract

Trp‐8 and Pro‐13 of the Rhodobacter capsulatus light‐harvesting (LH) I α polypeptide are highly conserved among LHI and LHII α proteins of several species of the Rhodospirillaceae. Exchange of Trp‐8 and Pro‐13 to other amino acyl residues similar in structure and/or hydrophobicity indicates that Trp‐8 is involved in the insertion of the LHI α polypeptide into the intracytoplasmic membrane (ICM). Pro‐13, however, seems not to participate in the integration process of the LHI α protein but seems to be important for stable insertion of the LHI β partner protein in the ICM.

Characterization of LHI- and LHI+ Rhodobacter capsulatus pufA mutants

“…The amino acid Pro is known as a helix-breaking residue. Therefore, one possible explanation of the results of our work and those of former studies (42) could be that the NH2 termini of the LHI at and I polypeptides are wound around each other and that the LHI a Pro-13 is needed for fixation of the LHI 3 partner polypeptide in the ICM. Deletion of Pro-13 or change of its relative position will change the normal tertiary structure of the LHI a NH2 terminus and therefore the interactions between the two LHI polypeptides.…”

“…The conserved amino acid Trp-8 probably is one of the sites necessary for insertion of the LHI a protein into the ICM (42). Carotenoids bound to Lys-Ile-Trp could function to prevent Trp-8 from oxidative degradation, thus establishing the role of Trp-8 in the membrane integration process of the LHI ao protein.…”

“…Arg-Ile-Trp and Lys-Ile-Trp stretches were conserved among numerous LH proteins from photosynthetic bacteria (6,50). It has been shown that the aromatic substructure of Trp is necessary for the integration of the LHI ao protein into the ICM (42). Exchange of Trp-8 of the LHI a protein of R. capsulatus to Ala reduced drastically the amount of the LHI a protein integrated into the membrane (42).…”

“…The aromatic structure of the Trp at position 8 seems to be of importance for the insertion of the LHI a polypeptide into the ICM (42). The conservative amino acid residue Pro-13, however, seems to be necessary for keeping the LHI 1 partner polypeptide in the membrane (42).…”

“…The mutations were carried out in a manner analogous to that described for deletion of the pufA and pufB genes (43). The mutations were confirmed by sequencing with appropriate primers (42)(43)(44). The plasmid constructions containing mutated puf operons were designated serially as pTPR13 to pTPR18 (Table 1).…”