Recombinant Lhca2 and Lhca3 Subunits of the Photosystem I Antenna System

In this study, two gene products (Lhca2 and Lhca3), encoding higher plants (Arabidopsis thaliana) Photosystem I antenna complexes, were overexpressed in bacteria and reconstituted in vitro with purified chloroplast pigments. The chlorophyll-xanthophyll proteins thus obtained were characterized by biochemical and spectroscopic methods. Both complexes were shown to bind 10 chlorophyll (a and b) molecules per polypeptide, Lhca2 having higher chlorophyll b content as compared to Lhca3. The two proteins differed for the number of carotenoid binding sites: two and three for Lhca2 and Lhca3, respectively. beta-carotene was specifically bound to Lhca3 in addition to the xanthophylls violaxanthin and lutein, indicating a peculiar structure of carotenoid binding sites in this protein since it is the only one so far identified with the ability of binding beta-carotene. Analysis of the spectroscopic properties of the two pigment proteins showed the presence of low energy absorption forms (red forms) in both complexes, albeit with different energies and amplitudes. The fluorescence emission maximum at 77 K of Lhca2 was found at 701 nm, while in Lhca3 the major emission was at 725 nm. Reconstitution of Lhca3 without Chl b reveals that Chl b is not involved in originating the low energy absorption forms of this complex. The present data are discussed in comparison to the properties of the recombinant Lhca1 and Lhca4 complexes and of the native LHCI preparation, previously analyzed, thus showing a comprehensive description of the gene products composing the Photosystem I light harvesting system of A. thaliana.