PSA3, a Protein on the Stromal Face of the Thylakoid Membrane, Promotes Photosystem I Accumulation in Cooperation with the Assembly Factor PYG7.

PSI is a large protein-pigment complex located in the thylakoid membrane in cyanobacteria, plants, and algae. Although the structure and components of PSI are well characterized, mechanisms that orchestrate its assembly are poorly understood. In this study, we discovered a novel nucleus-encoded protein, Photosystem I Assembly3 (PSA3), that is required for PSI accumulation. PSA3 is conserved among green photosynthetic eukaryotes but is lacking in cyanobacteria. Mutations in the psa3 gene cause the specific loss of PSI in Arabidopsis (Arabidopsis thaliana) and maize (Zea mays). Ribosome profiling and pulse-labeling analyses showed that chloroplast- encoded PSI subunits are synthesized at normal rates in psa3 mutants, indicating that PSA3 is involved in the biogenesis of PSI at a posttranslational step. PSA3 resides on the stromal face of the thylakoid membrane, where it is found in a complex that is slightly smaller than PSI. Structural predictions suggest that PSA3 binds a basic peptide in a manner that is sensitive to the oxidation state of Cys pairs flanking the predicted peptide binding groove. PSA3 and the previously described PSI biogenesis factor PYG7 interact in yeast two-hybrid and bimolecular fluorescence complementation assays, and they are found in thylakoid membrane complexes of similar size. These and other results indicate that PSA3 cooperates with PYG7 to promote the stable assembly of PSI, and that the PsaC subunit is likely to be the primary target of their action.