Coupling oxidative signals to protein phosphorylation via methionine oxidation in Arabidopsis

The mechanisms involved in sensing oxidative signalling molecules, such as H2O2, in plant and animal cells are not completely understood. In the present study, we tested the postulate that oxidation of Met (methionine) to MetSO (Met sulfoxide) can couple oxidative signals to changes in protein phosphorylation. We demonstrate that when a Met residue functions as a hydrophobic recognition element within a phosphorylation motif, its oxidation can strongly inhibit peptide phosphorylation in vitro. This is shown to occur with recombinant soybean CDPKs (calcium-dependent protein kinases) and human AMPK (AMP-dependent protein kinase). To determine whether this effect may occur in vivo, we monitored the phosphorylation status of Arabidopsis leaf NR (nitrate reductase) on Ser(534) using modification-specific antibodies. NR was a candidate protein for this mechanism because Met(538), located at the P + 4 position, serves as a hydrophobic recognition element for phosphorylation of Ser(534) and its oxidation substantially inhibits phosphorylation of Ser(534) in vitro. Two lines of evidence suggest that Met oxidation may inhibit phosphorylation of NR-Ser(534) in vivo. First, phosphorylation of NR at the Ser(534) site was sensitive to exogenous H2O2 and secondly, phosphorylation in normal darkened leaves was increased by overexpression of the cytosolic MetSO-repair enzyme PMSRA3 (peptide MetSO reductase A3). These results are consistent with the notion that oxidation of surface-exposed Met residues in kinase substrate proteins, such as NR, can inhibit the phosphorylation of nearby sites and thereby couple oxidative signals to changes in protein phosphorylation.