Identification of N- and C-terminal amino acids of Lhca1 and Lhca4 required for formation of the heterodimeric peripheral photosystem I antenna LHCI-730

Apoproteins of higher plant light-harvesting complexes (LHC) share considerable amino acid sequence identity/similarity. Despite this fact, they occur in different oligomeric states (i.e., monomeric, dimeric, and trimeric). As a step toward understanding the underlying structure requirements for different oligomerization behavior, we analyzed whether amino acids at the N- and C-termini of Lhca1 and Lhca4 are involved in the formation of the heterodimeric LHCI-730. Using altered proteins produced by deletion or site-directed mutagenesis for reconstitution, we were able to identify amino acids required for the assembly of LHCI-730. At the N-terminus of Lhca1, W4 is involved in heterodimerization. This interaction probably depends on aromatic properties because only replacement of W4 by F resulted in dimer formation, Also, at the C-terminus of Lhca1, W seems to play a crucial role for interaction with Lhca4. A detailed analysis by point mutants revealed the importance of an aromatic residue at position 185. One or more other amino acid(s) located downstream of position 188 may exert additional stabilizing, effects, presumably in a cooperative way. The scenario for Lhca4 is different. Dimerization broke down only after the deletion of the entire extrinsic N- or C-terminal region, demonstrating that the termini of Lhca4 are not involved in strong interactions with Lhca1 decisive for dimerization. At the N-terminus, dimerization was abolished after the removal of the same number of amino acids at which monomer formation failed. Site-specific mutagenesis of the amino acid decisive for LHC-formation in a deletion study demonstrated that its character is of no importance for dimerization and, therefore, that abolition of dimer formation may be the consequence of a loss in monomer formation. At the C-terminus of Lhca4, an even higher number of amino acids than required for monomer formation could be removed without the loss of dimerization. The decisive position is 1168, located in the third transmembrane region. Because all point mutants of 1168 in the full-length protein yielded dimers, failure of dimerization may be caused by either falling below a critical length of the polypeptide chain, resulting in the loss of too many weak interactions, or by too strong an impairment of Lhca4-folding. Interestingly, N- and C-terminal mutants of Lhca4 not able to form stable monomers formed stable dimers, indicating stabilization of labile monomeric complexes by the Lhca1 subunit in dimerization. Finally, the significance for dimer formation of amino acids in other parts of Lhca1 and Lhca4 which may be involved, besides the amino acids identified here in the specific assembly of the heterodimeric LHCI-730, is discussed. Their identification will result in a better understanding of structure characteristics determining the different oligomerization behavior of LHCs.