SeIR reverses Mical-mediated oxidation of actin to regulate F-actin dynamics

Actin's polymerization properties are markedly altered by oxidation of its conserved Met 44 residue. Mediating this effect is a specific oxidation-reduction (redox) enzyme, Mica!, that works with Semaphorin repulsive guidance cues and selectively oxidizes Met 44. We now find that this actin-regulatory process is reversible. Employing a genetic approach, we identified a specific methionine sulfoxide reductase (MsrB) enzyme SeIR that opposes Mical redox activity and Semaphorin-Plexin repulsion to direct multiple actin-dependent cellular behaviours in vivo. SeIR specifically catalyses the reduction of the R isomer of methionine sulfoxide (methionine-R-sulfoxide) to methionine, and we found that SeIR directly reduced Mical-oxidized actin, restoring its normal polymerization properties. These results indicate that Mical oxidizes actin stereospecifically to generate actin Met-44-R-sulfoxide (actin(Met(R)O-44)) and also implicate the interconversion of specific Met/Met(R)O residues as a precise means to modulate protein function. Our results therefore uncover a specific reversible redox actin regulatory system that controls cell and developmental biology.