Solution structure of the unbound, oxidized photosystem I subunit PsaC, containing [4Fe-4S] clusters F_A and F_B: A conformational change occurs upon binding to photosystem I

This work presents the three-dimensional NMR solution structure of recombinant, oxidized, unbound PsaC from Synechococcus sp. PCC 7002. Constraints are derived from homo- and heteronuclear one-, two- and three-dimensional ¹H and ¹⁵N NMR data. Significant differences are outlined between the unbound PsaC structure presented here and the available X-ray structure of bound PsaC as an integral part of the whole cyanobacterial PS I complex. These differences mainly concern the arrangement of the N- and C-termini with respect to the [4Fe-4S] core domain. In the NMR solution structure of PsaC the C-terminal region assumes a disordered helical conformation, and is clearly different from the extended coil conformation, which is one of the structural elements required to anchor PsaC to the PS I core heterodimer. In solution the N-terminus of PsaC is in contact with the pre-C-terminal region but slides in between the latter and the iron-sulfur core region of the protein. Together, these features result in a concerted movement of the N-terminus and pre-C-terminal region away from the F_A binding site, accompanied by a bending of the N-terminus. In comparison, the same terminal regions are positioned much closer to F_A and take up an anti-parallel β-sheet arrangement in PsaC bound to PS I. The conformational changes between bound and unbound PsaC correlate with the differences reported earlier for the EPR spectra of reduced F_A and F_B in bound versus unbound PsaC. The observed different structural features in solution are highly relevant for unraveling the stepwise assembly process of the stromal PsaC, PsaD and PsaE subunits to the PS I core heterodimer.