Xanthophyll binding sites of the CP29 (Lhcb4) subunit of higher plant photosystem II investigated by domain swapping and mutation analysis

The binding sites for xanthophylls in the CP29 antenna protein of higher plant Photosystem II have been investigated using recombinant proteins refolded in vitro. Despite the presence of three xanthophyll species CP29 binds two carotenoids per polypeptide. The localization of neoxanthin was studied producing a chimeric protein constructed by swapping the C-helix domain from CP29 to LHCII. The resulting holoprotein did not bind neoxanthin, confirming that the N1 site is not present in CP29. Neoxanthin in CP29 was, instead, bound to the L2 site, which is thus shown to have a wider specificity with respect to the homologous site L2 in LHCII. Lutein was found in the L1 site of CP29. For each site the selectivity for individual xanthophyll species was studied as well as its role in protein stabilization, energy transfer, and photoprotection. Putative xanthophyll binding sequences, identified by primary structure analysis as a stretch of hydrophobic residues including an acidic term, were analyzed by site-directed mutagenesis or, in one case, by deleting the entire sequence. The mutant proteins were unaffected in their xanthophyll composition, thus suggesting that the target motifs had little influence in determining xanthophyll binding, whereas hydrophobic sequences in the membrane-spanning helices are important.