Assembly of Light-Harvesting Systems

Abstract

SummaryPlastid development in ‘yellow-in-the-dark’ strains of Chlamydomonas reinhardtii provides a unique system to investigate assembly of light-harvesting complexes (LHC) in situ. The ultrastructure of the plastid in degreened cells exposed to only a few minutes of light at 38°C indicated that thylakoid membrane formation was initiated by expansion of the envelope. The kinetics of accumulation of LHCII were consistent with assembly of the complex concomitant with import of the apoproteins into the chloroplast. The initial fluorescence of newly formed LHCII was quenched within the first minute of greening as the energy transducing system also assembled within initial membranes. Most of the chlorophyll (Chl) in early greening cells was distributed in foci at the periphery of the chloroplast. In such cells, LHCII apoproteins were detected adjacent to the inner surface of the envelope by immunoelectron microscopy. Interestingly, LHCII apoproteins were also detected outside of the chloroplast in vacuoles, rather than in the chloroplast stroma, when synthesized in the absence of synthesis of Chl. Wall-deficient cw15 cells, grown in the light or dark at 28°C, contain large granules within these vacuoles that were immunoreactive to antibodies against LHCII apoproteins. In a Chl b-less strain, cbn1-113 arg2 y, which is an algal analogue of chlorina mutants of plants when grown in medium lacking acetate, LHCII apoproteins accumulated also in cytoplasmic vacuoles. The apoproteins were quantitatively recovered as the mature-sized species in each case, which suggested that even those outside of the chloroplast were processed. A possible interpretation of these results is that assembly of LHCII in the chloroplast envelope, by association of apoproteinswith Chl and xanthophylls, is required to prevent retrogrademovement of the proteins into the cytosol.