Structure of plant Photosystem I in a native assembly state

Abstract Plant Photosystem I (PSI) consists of at least 13 nuclear-encoded and four chloroplast-encoded protein subunits, acting as a sunlight-driven oxidoreductase. Here, we report the first structure of PSI assembly intermediate isolated from greening oat seedlings. The assembly state displays absence of at least eight subunits and lacks the photoreduction activity, which is attributed to an elimination of the plastocyanin (Pc) binding site, due to a missing subunit PsaF. The data shows that PsaF is a regulatory checkpoint that promotes the assembly, effectively coupling biogenesis to function.