Formylation facilitates the reduction of oxidized initiator methionines

Within a cell, protein-bound methionines can be oxidized by reactive oxygen species (ROS) or monooxygenases, and subsequently reduced by methionine sulfoxide reductases (Msrs). Methionine oxidation can result in structural damage or be the basis of functional regulation of enzymes. In addition to participating in redox reactions, methionines play an important role as the initiator residue of translated proteins where they are commonly modified at their α-amine group by formylation or acetylation. Here, we investigated how formylation and acetylation of initiator methionines impact their propensity for oxidation and reduction. We show that in vitro , N-terminal methionine residues are particularly prone to chemical oxidation, and that their modification by formylation or acetylation greatly enhances their subsequent enzymatic reduction by MsrA and MsrB. Concordantly, in vivo ablation of methionyl-tRNA formyltransferase (MTF) in E. coli increases the prevalence of oxidized methionines within synthesized proteins. We show that oxidation of formylated initiator methionines is detrimental in part because it obstructs their ensuing deformylation by peptide deformylase (PDF) and hydrolysis by methionyl aminopeptidase (MAP). Thus, by facilitating their reduction, formylation mitigates the misprocessing of oxidized initiator methionines. Classification: Biological Sciences; Biochemistry ### Competing Interest Statement The authors have declared no competing interest. * LC-MS/MS : liquid chromatography-tandem mass spectrometry SPROX : Stability of Proteins from Rates of Oxidation MObB : Methionine Oxidation by Blocking MTF : methionyl-tRNA formyltransferase PDF : peptide deformylase MAP : methionine aminopeptidase MsrA : methionine sulfoxide reductase A MsrB : methionine sulfoxide reductase B DTT : dithiothreitol IAA : iodoacetamide TFA : trifluoroacetic acid ACN : acetonitrile PSM : peptide-spectrum match