A seven transmembrane-integral methyltransferase catalysing N-terminal histidine methylation of lytic polysaccharide monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) are oxidative enzymes that help break down lignocellulose, making them highly attractive for improving biomass utilization in biotechnological purposes. The catalytically essential N-terminal histidine (His1) of LPMOs is post-translationally modified by methylation in filamentous fungi to protect them from auto-oxidative inactivation, however, the responsible methyltransferase enzyme is unknown. Using mass-spectrometry-based quantitative proteomics in combination with systematic CRISPR/Cas9 knockout screening in Aspergillus nidulans, we identified the N-terminal histidine methyltransferase (NHMT) encoded by the gene AN4663. Targeted proteomics confirmed that NHMT was solely responsible for His1 methylation of LPMOs. NHMT is predicted to encode a unique seven-transmembrane segment anchoring a soluble methyltransferase domain. Co-localization studies showed endoplasmic reticulum residence of NHMT and co-expression in the industrial production yeast Komagataella phaffii with LPMOs resulted in His1 methylation of the LPMOs. This demonstrates the biotechnological potential of recombinant production of proteins and peptides harbouring this unique post-translational modification.