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.