Solution Structures of the Core Light-harvesting α and β Polypeptides from Rhodospirillum rubrum: Implications for the Pigment–Protein and Protein–Protein Interactions Original Research Article

We have determined the solution structures of the core light-harvesting (LH1) α and β-polypeptides from wild-type purple photosynthetic bacterium Rhodospirillum rubrum using multidimensional NMR spectroscopy. The two polypeptides form stable α helices in organic solution. The structure of α-polypeptide consists of a long helix of 32 amino acid residues over the central transmembrane domain and a short helical segment at the N terminus that is followed by a three-residue loop. Pigment-coordinating histidine residue (His29) in the α-polypeptide is located near the middle of the central helix. The structure of β-polypeptide shows a single helix of 32 amino acid residues in the membrane-spanning region with the pigment-coordinating histidine residue (His38) at a position close to the C-terminal end of the helix. Strong hydrogen bonds have been identified for the backbone amide protons over the central helical regions, indicating a rigid property of the two polypeptides. The overall structures of the R. rubrum LH1 α and β-polypeptides are different from those previously reported for the LH1 β-polypeptide of Rhodobacter sphaeroides, but are very similar to the structures of the corresponding LH2 α and β-polypeptides determined by X-ray crystallography. A model constructed for the structural subunit (B820) of LH1 complex using the solution structures reveals several important features on the interactions between the LH1 α and β-polypeptides. The significance of the N-terminal regions of the two polypeptides for stabilizing both B820 and LH1 complexes, as clarified by many experiments, may be attributed to the interactions between the short N-terminal helix (Trp2-Gln6) of α-polypeptide and a GxxxG motif in the β-polypeptide.