Abstract
In plants, the photosystem I (PSI) core complex stably associates with its light-harvesting chlorophyll a/b complex I (LHCI) to form the PSI-LHCI supercomplex. The vascular plant PSI core complex associates with four distinct LHCI subunits, whereas that of the green alga Chlamydomonas reinhardtii binds nine distinct LHCI subunits (LHCA1-LHCA9). The stoichiometry and configuration of these LHCI subunits in the PSI-LHCI supercomplex of C. reinhardtii remain controversial. Here, we determined the stoichiometry of the nine distinct LHCI subunits relative to PSI subunits through uniform labeling of total proteins using 14C. We separated the nine LHCI polypeptides by three different sodium dodecyl sulfate-polyacrylamide gel electrophoresis systems. Our data revealed that the PSI-LHCI supercomplex contains two LHCA1 proteins and one of each of the other eight LHCI subunits. Subsequently, we identified their cross-linked products by immunodetection and mass spectrometry to determine the configuration of the 10 LHCI subunits within the PSI-LHCI supercomplex. Furthermore, analyses of PSI-LHCI complexes isolated from ΔLHCA2 and ΔLHCA5 mutants and oligomeric LHCI from a PSI-deficient (ΔpsaA/B) mutant provided supporting evidence for the LHCI subunit configuration. In conclusion, eight LHCI subunits bind to the PSI core at the site of PSAF subunit in two layers: LHCA1-LHCA8-LHCA7-LHCA3 from PSAG to PSAK, in the inner layer, and LHCA1-LHCA4-LHCA6-LHCA5 in the outer layer. The other two LHCI subunits, LHCA2 and LHCA9, bind PSAB between PSAG and PSAH, PSAG-LHCA9-LHCA2-PSAH. Our study provides new insights into the LHCI configuration linked to the PSI core.
Highlights
In plants, the photosystem I (PSI) core complex stably associates with its light-harvesting chlorophyll a/b complex I (LHCI) to form the PSI-LHCI supercomplex
We isolated the PSI-LHCI supercomplexes from cells grown in medium containing 40 μm or 17 mm acetate to assess whether the concentration of acetate in the medium affected the accumulation of LHCI subunits
We confirmed that the polypeptide compositions of the two PSI-LHCI preparations isolated from the cells grown in the medium containing acetate at different concentrations were similar (Supplemental Fig. S1A), indicating that the acetate concentration in the growth medium did not substantially affect the stoichiometry of LHCI subunits in the PSI-LHCI supercomplex
Summary
The photosystem I (PSI) core complex stably associates with its light-harvesting chlorophyll a/b complex I (LHCI) to form the PSI-LHCI supercomplex. It was reported that nine distinct LHCI subunits are stoichiometrically associated with the PSI core (Drop et al, 2011) when the complex was isolated from low-light photoautotrophic growth conditions. In the absence of chlorophyll (Chl) b, six LHCI subunits (LHCA1, LHCA2, LHCA3, LHCA7, LHCA8, and LHCA9) remain associated with the purified PSI subcomplex, whereas three LHCI subunits (LHCA4– LHCA6) are lost during PSI preparation, suggesting that LHCA4 to LHCA6 are present in the outer layer and, are more loosely associated with the PSI core (Bujaldon et al, 2017) We report both the stoichiometry and configuration of nine distinct LHCI subunits in the PSI-LHCI supercomplex of C. reinhardtii. The resulting structural model of LHCI subunit configuration was confirmed by the biochemical analyses of two LHCI mutants deficient in either LHCA2 or LHCA5 and a PSI-deficient mutant
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