The Photoprotective Protein PsbS from Green Microalga : The Amino Acid Sequence, 3D Structure and Probable pH-Sensitive Residues.

PsbS is one of the key photoprotective proteins, ensuring the tolerance of the photosynthetic apparatus (PSA) of a plant to abrupt changes in irradiance. Being a component of photosystem II, it provides the formation of quenching centers for excited states of chlorophyll in the photosynthetic antenna with an excess of light energy. The signal for "turning on" the photoprotective function of the protein is an excessive decrease in pH in the thylakoid lumen occurring when all the absorbed light energy (stored in the form of transmembrane proton potential) cannot be used for carbon assimilation. Hence, lumen-exposed protonatable amino acid residues that could serve as pH sensors are the essential components of PsbS-dependent photoprotection, and their pK values are necessary to describe it. Previously, calculations of the lumen-exposed protonatable residue pK values in PsbS from spinach were described in the literature. However, it has recently become clear that PsbS, although typical of higher plants and charophytes, can also provide photoprotection in green algae. Namely, the stress-induced expression of PsbS was recently shown for two green microalgae species: and . Therefore, we determined the amino acid sequence and modeled the three-dimensional structure of the PsbS from , as well as calculated the pK values of its lumen-exposed protonatable residues. Despite significant differences in amino acid sequence, proteins from and have similar three-dimensional structures. Along with the other differences, one of the two pH-sensing glutamates in PsbS from (namely, Glu-173) has no analogue in protein. Moreover, there are only four glutamate residues in the lumenal region of the protein, while there are eight glutamates in . However, our calculations show that, despite the relative deficiency in protonatable residues, at least two residues of PsbS can be considered probable pH sensors: Glu-87 and Lys-196.