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Methionine sulfoxide reductase reverses oxidation of the nucleotide exchange factor Fes1 to regulate cytoplasmic Hsp70 chaperone cycle

Methionine residues in proteins are targets of oxidation by reactive oxygen species (ROS). Selective oxidation of methionine to methionine sulfoxide (MetO) can allow for reversible, beneficial modification of proteins to limit cell damage during conditions of excess ROS. Methionine sulfoxide reductases (Msrs) are conserved enzymes that reduce MetO residues in proteins, serving to both limit the over‐oxidation of protein methionines, and to remove MetO adducts when stress subsides. Although loss of Msr function has been implicated in neurodegenerative disorders and aging, it is unclear how Msr activities influence these disease pathologies because the vast majority of their physiological substrates remain unknown.