The double mutation ΔL6MW241F in PsbO, the photosystem II manganese stabilizing protein, yields insights into the evolution of its structure and function.

The W241F mutation in spinach manganese-stabilizing protein (PsbO) decreases binding to photosystem II (PSII); its thermostability is increased and reconstituted activity is lower [Wyman et al. (2008) Biochemistry 47, 6490-6498]. The results reported here show that W241F cannot adopt a normal solution structure and fails to reconstitute efficient Cl(-) retention by PSII. An N-terminal truncation of W241F, producing the ΔL6MW241F double mutant that resembles some features of cyanobacterial PsbO, significantly repairs the defects in W241F. Our data suggest that the C-terminal F→W mutation likely evolved in higher plants and green algae in order to preserve proper PsbO folding and PSII binding and assembly, which promotes efficient Cl(-) retention in the oxygen-evolving complex.