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

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 N{pi}-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 N{pi}-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.