Probing the structure of Lhca3 by mutation analysis.

Lhc proteins constitute a family of transmembrane proteins which share homology in sequence and similarity in the general organisation although members can be strongly differentiated such as in the case of PsbS and ELIPs. In this work, we report on the structure of Lhca3, a pigment-protein subunit component of the antenna system of higher plants Photosystem I, through the effect of point mutations in critical sites. Based on the structure of PSI-LHCI (Ben Shem et al., PDB file 1QZV remark 999) it has been suggested that Lhca3 may have different folding as compared to other members of the Lhc family. In particular, it was proposed that the two central helices may be swapped and chlorophylls in sites 1013 and 1023 are not present. This different folding would imply that the chlorophylls coordinated to the two central helices have different ligands in Lhca3 with respect to the other Lhc complexes. The structural model was tested by substituting the putative binding residues with residues unable to coordinate chlorophyll and the spectroscopic properties of the individual pigments were used as structural probes. The results indicate that Lhca3 folds in the same way as the other antenna proteins. Moreover, the low-energy absorption form originates from interaction between chlorophylls in site 1015 and 1025, like for the other PSI antenna subunits. Evidence is also shown for the presence in Lhca3 of chlorophylls in sites 1013 and 1023.