The Role of the Envelope in Assembly of Light-Harvesting Complexes in the Chloroplast: Distribution of Lhcp between Chloroplast and Vacuoles during Chloroplast Development in Chlamydomonas reinhardtii

Abstract

The formation of thylakoid membranes during chloroplast development is crucial for support of the biosphere and remains a major question in plant cell biology. Biogenesis of functional photosystems involves assembly of light-harvesting complexes (LHCI and LHCII) and their association with PSI and PSII core complexes. We focused on assembly of LHCII as a means to probe for the context in which biogenesis of the membrane occurs. LHCPs, the apoproteins of LHCII, are the major proteins of the membrane and thereby provide a strong signal for analytical studies. These proteins are synthesized on cytoplasmic ribosomes as precursors, imported into the chloroplast, and processed by proteases in the stroma. The following are likely events leading to interaction of LHCPs with pigments. Photoreduction of protochlorophyllide (Pchlide) by light-dependent NADPH:Pchlide oxido-reductase (POR) produces chlorophyllide a (Chlide a). Binding of Pchlide to POR apparently causes release of Chlide [1]. The site at which this reaction occurs has not been established, but POR is an integral membrane protein in C. reinhardtii and possibly resides in the envelope. Fluorescence of Chlide a is quenched within seconds after release from the enzyme [2], which may result from interaction with xanthophylls in the membrane environment. Chlide a is esterified with geranylgeranyl (GG) diphosphate and normally reduced to the phytyl ester. During rapid greening of C. reinhardtii y1 cells at 38°C, however, incorporation of Chl into LHCII precludes substantial reduction of the side-chain [3]. Because lutein--the major xanthophyll in LHCII--is abundant in greening cells, assembly of LHCII under these conditions appears to be limited by synthesis of Chl.