A PSA4-YCF4 complex promotes assembly of the photosystem I reaction center in Arabidopsis.

Photosystem I (PSI) is one of nature's most intricate and efficient nanomachines driving photosynthetic electron transfer. Its biogenesis requires a sophisticated, multi-step assembly pathway coordinated by a suite of nucleus- and plastid-encoded factors, yet the precise molecular mechanisms governing the earliest and most critical steps regarding the formation of its reaction center PsaA-PsaB remain poorly understood. Here, we have identified and characterized PSA4 (Photosystem I Assembly 4), a thylakoid membrane protein conserved in land plants and green algae. PSA4 physically interacts with the conserved assembly factor YCF4 to form a functional complex. Loss of PSA4 drastically destabilizes YCF4 and impairs the normal accumulation of PSI. In vivo pulse-labeling demonstrates that the PSA4-YCF4 complex is specifically required for the efficient synthesis of the PSI reaction center subunits PsaA and PsaB, while their translation initiation appears unaffected. Topology analyses reveal that both the N- and C-termini of YCF4 face the thylakoid lumen, where they directly interact with four proximal luminal loops of the PsaA/PsaB heterodimer. Based on these findings, we propose that PSA4 and YCF4 form a dedicated complex that promotes the assembly of the PsaA/PsaB heterodimer during the early steps of PSI biogenesis in chloroplasts.