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Abstract
BackgroundIncreasing structural and biochemical evidence suggests that post-translational methionine oxidation of proteins is not just a result of cellular damage but may provide the cell with information on the cellular oxidative status. In addition, oxidation of methionine residues in key regulatory proteins, such as calmodulin, does influence cellular homeostasis. Previous findings also indicate that oxidation of methionine residues in signaling molecules may have a role in stress responses since these specific structural modifications can in turn change biological activities of proteins.FindingsHere we use tandem mass spectrometry-based proteomics to show that treatment of Arabidopsis thaliana cells with a non-oxidative signaling molecule, the cell-permeant second messenger analogue, 8-bromo-3,5-cyclic guanosine monophosphate (8-Br-cGMP), results in a time-dependent increase in the content of oxidised methionine residues. Interestingly, the group of proteins affected by cGMP-dependent methionine oxidation is functionally enriched for stress response proteins. Furthermore, we also noted distinct signatures in the frequency of amino acids flanking oxidised and un-oxidised methionine residues on both the C- and N-terminus.ConclusionsGiven both a structural and functional bias in methionine oxidation events in response to a signaling molecule, we propose that these are indicative of a specific role of such post-translational modifications in the direct or indirect regulation of cellular responses. The mechanisms that determine the specificity of the modifications remain to be elucidated.
Highlights
Increasing structural and biochemical evidence suggests that post-translational methionine oxidation of proteins is not just a result of cellular damage but may provide the cell with information on the cellular oxidative status
Given both a structural and functional bias in methionine oxidation events in response to a signaling molecule, we propose that these are indicative of a specific role of such post-translational modifications in the direct or indirect regulation of cellular responses
The debate of whether methionine (Met) oxidation of proteins is a purely chemical consequence of cellular oxidative damage or a protective mechanism against oxidative damage, or a post-translational modification that can act as a specific cellular signal and/or response, is ongoing [1,2,3]
Summary
Given both a structural and functional bias in methionine oxidation events in response to a signaling molecule, we propose that these are indicative of a specific role of such post-translational modifications in the direct or indirect regulation of cellular responses. We noted an increase in the total number of peptides containing residues of oxidised Met after cGMP treatment from 221 to 633 and 1451 at 0, 30 and 60 minutes, respectively (Figure 3A and Additional file 2, tab “AF2”). These numbers represent 1.4%, 19.4% and 13%, respectively, of the total number of peptides identified at each time point. The total Met oxidised peptides correspond to 34 (at 0’), 136 (at 30’) and 281 (at 60’) Arabidopsis proteins, from which 10, 94, and 224 identified oxidized Met proteins are unique for each time-point, respectively (Figure 3B) This finding implies either that cGMP-dependent Met modifications are reversible and/or that some of the modified proteins have undergone proteolysis.
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