Protein-repairing methionine sulfoxide reductases in photosynthetic organisms: gene organization, reduction mechanisms, and physiological roles.

Methionine oxidation to methionine sulfoxide (MetSO) is reversed by two types of methionine sulfoxide reductases (MSRs), A and B, specific to the S- and R-diastereomers of MetSO, respectively. MSR genes are found in most organisms from bacteria to human. In the current review, we first compare the organization of the MSR gene families in photosynthetic organisms from cyanobacteria to higher plants. The analysis reveals that MSRs constitute complex families in higher plants, bryophytes, and algae compared to cyanobacteria and all non-photosynthetic organisms. We also perform a classification, based on gene number and structure, position of redox-active cysteines and predicted sub-cellular localization. The various catalytic mechanisms and potential physiological electron donors involved in the regeneration of MSR activity are then described. Data available from higher plants reveal that MSRs fulfill an essential physiological function during environmental constraints through a role in protein repair and in protection against oxidative damage. Taking into consideration the expression patterns of MSR genes in plants and the known roles of these genes in non-photosynthetic cells, other functions of MSRs are discussed during specific developmental stages and ageing in photosynthetic organisms.