1.8 Å resolution structure of the cyanobacterial Photosystem I assembly intermediate lacking cytoplasmic subunits

Despite the detailed knowledge of photosystem I structure, the biogenesis of this energy-converting complex remains poorly understood. Here, we provide a 1.83 Å resolution model of a cyanobacterial PSI assembly intermediate isolated from a PsaC-less mutant. Our structure revealed the association of all small membrane subunits with the central PsaA/PsaB heterodimer except PsaL, which was missing together with PsaD and PsaE. The intermediate contained nearly all pigments and cofactors as the mature complex including the iron-sulfur cluster at the FX site, which was found incomplete; possibly due to conformational changes of PsaB. The data contravene previous assembly models initiated by the formation of PsaA/PsaB heterodimer and ending by incorporation of small membrane subunits. They rather support a parallel assembly of PsaA, PsaB, and PsaC, which is essential for the correct FX formation, while stable binding of small membrane subunits to the PsaA/PsaB heterodimer is mostly independent of the cytoplasmic subunits.