Identification and Characterization of an Assembly Intermediate Subcomplex of Photosystem I in the Green Alga Chlamydomonas reinhardtii

ArticleinJournal of Biological Chemistry 285(26):20072-9 · April 2010with7 Reads
Impact Factor: 4.57 · DOI: 10.1074/jbc.M109.098954 · Source: PubMed

    Abstract

    Photosystem I (PSI) is a multiprotein complex consisting of the PSI core and peripheral light-harvesting complex I (LHCI)
    that together form the PSI-LHCI supercomplex in algae and higher plants. The supercomplex is synthesized in steps during which
    12–15 core and 4–9 LHCI subunits are assembled. Here we report the isolation of a PSI subcomplex that separated on a sucrose
    density gradient from the thylakoid membranes isolated from logarithmic growth phase cells of the green alga Chlamydomonas reinhardtii. Pulse-chase labeling of total cellular proteins revealed that the subcomplex was synthesized de novo within 1 min and was converted to the mature PSI-LHCI during the 2-h chase period, indicating that the subcomplex was an
    assembly intermediate. The subcomplex was functional; it photo-oxidized P700 and demonstrated electron transfer activity.
    The subcomplex lacked PsaK and PsaG, however, and it bound PsaF and PsaJ weakly and was not associated with LHCI. It seemed
    likely that LHCI had been integrated into the subcomplex unstably and was dissociated during solubilization and/or fractionation.
    We, thus, infer that PsaK and PsaG stabilize the association between PSI core and LHCI complexes and that PsaK and PsaG bind
    to the PSI core complex after the integration of LHCI in one of the last steps of PSI complex assembly.