Secreted protein methyltransferase METTL9 catalyzes N π-histidine methylation of extracellular plasma proteins

Summary Post-translational modifications (PTMs) of proteins alter their biophysical properties, thus affecting their activity, localization and interactions. These reactions are principally intracellular events; however, to date, only phosphorylation has been shown to occur within the extracellular space. Here, we identified METTL9 as the first secreted methyltransferase responsible for Nπ-histidine methylation. METTL9 undergoes N-linked glycosylation, thereby forming dimers via disulfide bonds. Using a split-luciferase complementary assay, we revealed that N-glycosylated METTL9 is secreted extracellularly via the ER-Golgi pathway. Endogenous METTL9 is highly expressed in HL60 cells during neutrophil-like differentiation and secreted extracellularly. METTL9 catalyzes Nπ-methylhistidine formation in plasma proteins, in which the thyroxine transporter transthyretin (TTR) and copper transporter ceruloplasmin are identified as substrates for methylation in vitro. Both methylations occur at the His-x-His motif, a recognition sequence for METTL9, and TTR methylation decreases its binding affinity to zinc. Our results establish that histidine methylation is the second extracellular PTM, following phosphorylation. Competing Interest Statement The authors have declared no competing interest.