Interactions stabilizing the structure of the core light-harvesting complex (LH1) of photosynthetic bacteria and its subunit (B820)

Reconstitution experiments with a chemically synthesized core light-harvesting (LH1) beta-polypeptide analogue having 3-methylhistidine instead of histidine in the position that normally donates the coordinating ligand to bacteriochlorophyll (Bchl) have provided the experimental data needed to assign to B820 one of the two possible alphabeta.2Bchl pairs that are observed in the crystal structure of LH2 from Phaeospirillum (formerly Rhodospirillum) molischianum, the one with rings III and V of Bchl overlapping. Consistent with the assigned structure, experimental evidence is provided to show that significant stabilizing interactions for both the subunit complex (B820) and LH1 occur between the N-terminal regions of the alpha- and beta-polypeptides. On the basis of the results with the chemically synthesized polypeptides used in this study, along with earlier results with protease-modified polypeptides, mutants, and chemically synthesized polypeptides, the importance of a stretch of 9-13 amino acids at the N-terminal end of the alpha- and beta-polypeptides is underscored. A progressive loss of interaction with the LH1 beta-polypeptide was found as the first three N-terminal amino acids of the LH1 alpha-polypeptide were removed. The absence of the N-terminal formylmethionine (fMet), or conversion of the sulfur in this fMet to the sulfoxide, resulted in a decrease in LH1 formation. In addition to the removal of fMet, removal of the next two amino acids also resulted in a decrease in K(assoc) for B820 formation and nearly eliminated the ability to form LH1. It is suggested that the first three amino acids (fMetTrpArg) of the LH1 alpha-polypeptide of Rhodospirillum rubrum form a cluster that is most likely involved in close interaction with the side chain of His -18 (see Figure 1 for numbering of amino acids) of the beta-polypeptide. The results provide evidence that the folding motif of the alpha- and beta-polypeptides in the N-terminal region observed in crystal structures of LH2 is also present in LH1 and contributes significantly to stabilizing the complex.