Evidence That the FX Domain in Photosystem I Interacts with the Subunit PsaC: Site-Directed Changes in PsaB Destabilize the Subunit Interaction in Chlamydomonas reinhardtii

The highly conserved amino acid sequence PCDGPGRGGTC in both photosystem I reaction center core proteins PsaA and PsaB has been predicted to contribute the four cysteine ligands for coordination of the 4Fe-4S iron-sulfur cluster FX, and we have proposed a working model for the binding of PsaC to this domain of the reaction center core heterodimer [Rodday et al. (1993) Photosynth. Res. 36, 1-9]. We have investigated structure-function relationships between this domain and the PsaC subunit by site-directed mutagenesis of the conserved prolines P560 and P564, and the charged residues D562 and R566 in the eucaryotic alga Chlamydomonas reinhardtii. The D562N and R566E mutants did not accumulate the PsaA and PsaB reaction center proteins, indicating that these residues are essential for the stable assembly of photosystem I. The P560A, P560L, and P564L mutants accumulated functional reaction centers but showed an impaired interaction between the reaction center core complex and the PsaC subunit. We observed that the reaction centers of the proline mutants dissociated more readily in urea, and reconstitution of the mutant core preparations using PsaC and Fe-S cluster insertion protocols in vitro were incomplete. We suggest that P560 and D562 contribute to the stability of the FX cluster, most likely by providing essential hydrogen bonding to the C561 ligand. The data obtained from the P564 and R566 replacements provide direct evidence that the intercysteinyl region in PsaB is a domain involved in the interaction between PsaC and the reaction center core.