PsbS structure, sequence, location of active Glu-1 and Glu-2 and intradimer hydrogen bond interactions. a Structure of spinach PsbS (PDB-ID 4RI220), highlighting the active Glu (E69 and E173 in yellow). Part of the stromal loop between TM2 and TM3 is not resolved in the X-ray structure. b Amino acid sequences of spinach (S. oleracea) and P. patens PsbS, with the two active Glu highlighted in red. c X-ray structure of the PsbS dimer with the active Glu-1 (E69, in yellow) and Glu-2 (E173, in orange). d Detail of dimerization interface viewed from the luminal side. Helix H2 containing Glu-2 (E173) connects with the luminal loop containing Glu-1 (E69) of the adjacent monomer, via hydrogen bonds from the Y75 backbone amide to the E173 side chain and the I74 backbone amide to the E173 backbone carbonyl. e Schematic picture of the inter-monomer stabilizing interactions at the luminal side, involving Glu-2 (E173) in helix H2 and the 310 helix/luminal loop containing Glu-1 (E69).