Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: July 18, 2007
Publication Date: N/A
Technical Abstract: Protein phosphorylation can affect the activity, stability or localization of a protein and as result plays a broad role in regulation of processes ranging from metabolism to control of plant growth and development. One aspect of current interest in our lab is how protein kinases target their substrates and what mechanisms regulate the phosphorylation process in vivo. In particular, we are interested in what role reactive oxygen species (ROS), such as H2O2, may play. Our recent results suggest that reversible oxidation of methionine (Met) to Met sulfoxide (MetSO) may be a new mechanism to couple oxidative signals to protein phosphorylation. This notion initially arose from studies of the motifs recognized by calcium dependent protein kinases (CDPKs) and SNF1-related protein kinases (SnRK1s), which are closely related kinases that play a major role in phosphorylation of metabolic enzymes. Studies with synthetic peptide substrates demonstrated that these kinases target the canonical motif: '-x-Basic-x-x-Ser/Thr-x-x-x-', where ‘x’ is any amino acid and ' is a hydrophobic residue. We observed that when Met is the essential hydrophobic residue, its oxidation to MetSO strongly inhibits phosphorylation in vitro. This likely occurs because the MetSO side chain is hydrophilic rather than hydrophobic. That this effect may also occur in vivo is suggested by the observation that transgenic Arabidopsis plants over expressing a peptide Met sulfoxide reductase (PMSRA3), which reduces MetSO back to Met, have increased phosphorylation of numerous cellular proteins as revealed by ProQ Diamond Phosphoprotein staining of 2-DE gels. Preliminary MALDI-TOF MS analysis or immunoblotting with specific antibodies has identified hsp70, nitrate reductase, and chloroplast elongation factor EF-Tu as three of the phosphoproteins that are sensitive to oxidative signals in vivo. Furthermore, in silico analysis predicts numerous proteins may be dually regulated by reversible phosphorylation and Met oxidation. We are speculating that the propensity of Met residues to be reversibly oxidized to MetSO may have been exploited in nature to produce phosphorylation sites in certain proteins that would be responsive to oxidative signals. This general mechanism could be of broad significance as oxidative signals are generated during normal growth and also in response to most biotic and abiotic stresses.