Amino Acids in the Second Transmembrane Helix of the Lhca4 Subunit Are Important for Formation of Stable Heterodimeric Light-harvesting Complex LHCI-730

Photosynthetic light-harvesting complexes (LHCs) are assembled from apoproteins (Lhc proteins) and non-covalently attached pigments. Despite a considerable amino acid sequence identity, these proteins differ in their oligomerization behavior. To identify the amino acid residues determining the heterodimerization of Lhca1 and Lhca4 to form LHCI-730, we mutated the poorly conserved second transmembrane helix of the two subunits. Mutated genes were expressed in Escherichia coli and the resultant proteins were refolded in vitro and subsequently analyzed by gel electrophoresis. Replacement of the entire second helix in Lhca4 by the one of Lhca3 abolished heterodimerization, whereas it had no effect in Lhca1. Individual replacement of three amino acid clusters in Lhca4 that deviate from the corresponding sequence of Lhca3, demonstrated their contribution to Lhca1-Lhca4 dimerization. Further dissection by mutation of individual amino acid residues in Lhca4 showed the importance of a serine, phenylalanine, and histidine (S88, F95, H99) for LHCI-730 assembly. Alignment of consensus sequences of the Lhc proteins demonstrated that these amino acids are predominantly unique in Lhca4 at the relevant positions. Construction of a homology model based on the high-resolution structure of LHCII and superimposing these models onto the photosystem I structure suggested an orientation of S88, F95, and H99 toward the third transmembrane helix of Lhca1. Since some of the amino acids are too far apart from their nearest neighbors in Lhca1 for a direct interaction, different modes of interaction are discussed. Finally, by quantifying chlorophylls bound to monomeric LHC obtained with the H99 mutant, we identified this amino acid as a further chlorophyll binding site.