Asp157 is required for the function of PsbO, the photosystem II manganese stabilizing protein

PsbO, the photosystem II manganese stabilizing protein, contains an aspartate residue [Asp157 (spinach numbering)], which is highly conserved in eukaryotic and prokaryotic PsbOs. The homology model of the PSII-bound conformation of spinach PsbO presented here positions Asp157 in the large flexible loop of the protein. We have characterized site-directed mutants (D157N, D157E, and D157K) of spinach PsbO that were rebound to PsbO-depleted PSII to probe the role of Asp157. Structural data revealed that PsbO Asp157 mutants exhibit near-wild-type solution structure at 25 degrees C, but functional analyses of the mutants showed that these are the first genetically modified PsbO proteins from spinach that combine wild-type PSII binding behavior with significantly impaired O(2) evolution activity; all of the mutants reconstituted approximately 30% of control O(2) evolution activity. PsbO Asp157 has been proposed to be a part of a putative H(2)O/H(+) channel that links the active site of the oxygen-evolving complex with the lumen [De Las Rivas, J., and Barber, J. (2004) Photosynth. Res. 81, 329-343]. Unsuccessful attempts to use chemical rescue to enhance the activity restored by Asp157 mutants could indicate that this residue is not involved in a proton transfer network. It is shown, however, that these mutants are deficient in restoring efficient Cl(-) retention by PSII.