Identification and Characterization of an Assembly Intermediate Subcomplex of Photosystem I in the Green Alga Chlamydomonas reinhardtii
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.
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ABSTRACT: Photosystem I, an integral membrane and multi-subunit complex, catalyzes the oxidation of plastocyanin and the reduction of ferredoxin by absorbed light energy. Photosystem I participates in photosynthetic acclimation processes by being involved in cyclic electron transfer and state transitions for sustaining efficient photosynthesis. The photosystem I complex is highly conserved from cyanobacteria to higher plants and contains the light-harvesting complex and the reaction center complex. The assembly of the photosystem I complex is highly complicated and involves the concerted assembly of multiple subunits and hundreds of cofactors. A suite of regulatory factors for the assembly of photosystem I subunits and cofactors have been identified that constitute an integrative network regulating PSI accumulation. This review aims to discuss recent findings in the field relating to how the photosystem I complex is assembled in oxygenic organisms. This article is a Special Issue entitled: Chloroplast biogenesis. This article is part of a Special Issue entitled: Chloroplast Biogenesis. Copyright © 2015. Published by Elsevier B.V.Biochimica et Biophysica Acta (BBA) - Bioenergetics 01/2015; DOI:10.1016/j.bbabio.2014.12.011 · 4.83 Impact Factor
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ABSTRACT: Photosystem I (PS I) is a multisubunit membrane protein complex that functions as a light-driven plastocyanin-ferredoxin oxidoreductase. The PsbP-domain Protein 1 (PPD1, At4g15510) is located in the thylakoid lumen of plant chloroplasts and is essential for photoautotrophy, functioning as a PS I assembly factor. In this work, RNAi was used to suppress PPD1 expression, yielding mutants displaying a range of phenotypes with respect to PS I accumulation and function. These PPD1 RNAi mutants showed a loss of assembled PS I which was correlated with loss of the PPD1 protein. In the most severely affected PPD1 RNAi lines, the accumulated PS I complexes exhibited defects in electron transfer from plastocyanin to the oxidized reaction center P700+. The defects in PS I assembly in the PPD1 RNAi mutants also had secondary effects with respect to the association of light-harvesting antenna complexes to PS I. Due to the imbalance in photosystem function in the PPD1 RNAi mutants, light-harvesting complex II (LHC II) associated with and acted as an antenna for the PS I complexes. These results provide new evidence for the role of PPD1 in PS I biogenesis, particularly as a factor essential for proper assembly of the lumenal portion of the complex.Journal of Biological Chemistry 07/2014; 289(34). DOI:10.1074/jbc.M114.589085 · 4.60 Impact Factor
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ABSTRACT: Photosystem I (PSI) is a large pigment-protein complex and one of the two photosystems that drive electron transfer in oxygenic photosynthesis. We identified a nuclear gene required specifically for the accumulation of PSI in a forward genetic analysis of chloroplast biogenesis in maize. This gene, designated PSA2, belongs to the GreenCut gene set, a group of genes found in green algae and plants but not in non-photosynthetic organisms. Disruption of the psa2 ortholog in Arabidopsis likewise results in the specific loss of PSI proteins. PSA2 harbors a conserved domain found in DnaJ chaperones, where it has been shown to form a zinc finger and to have protein disulfide isomerase activity. Accordingly, PSA2 exhibits protein disulfide reductase activity in vitro. PSA2 localizes to the thylakoid lumen and is found in a ~250 kDa complex harboring the peripheral PSI protein PsaG, but lacking several core PSI subunits. PSA2 mRNA is coexpressed with mRNAs encoding various proteins involved in the biogenesis of the photosynthetic apparatus, with peak expression preceding that of genes encoding structural components. PSA2 protein abundance is not decreased in the absence of PSI, but is reduced in the absence of the PSI assembly factor YCF3. These findings suggest that a complex harboring PSA2 and PsaG mediates thiol transactions in the thylakoid lumen that are important for the assembly of PSI.Journal of Biological Chemistry 09/2014; 289(44). DOI:10.1074/jbc.M114.587758 · 4.60 Impact Factor