METTL1 Interacts with XPO5 to Modulate Pre-miRNA Export

tRNA methylguanosine methyltransferase 1 (METTL1) is the prominent m7G writer protein in yeast and human cells. While several studies documented the transcriptome-wide distributions of METTL1-installed m7G, very little is known about the protein-protein interactions (PPI) involving METTL1. To fill in this knowledge gap, we adopted an engineered ascorbate peroxidase (APEX)-based proximity labeling, followed by LC-MS/MS analysis, to investigate the METTL1 interactome in HEK293T cells. We identified more than 70 unique proteins enriched in the proximity proteome of METTL1, and verified METTL1’s interaction with one of these proteins, exportin 5 (XPO5) through co-immunoprecipitation followed by immunoblot analysis. We also revealed that genetic ablation of METTL1 led to elevated distribution of XPO5 in the cytosol, thereby enhancing pre-miRNA export and miRNA maturation. Mechanistically, METTL1 promotes ERK-mediated phosphorylation of XPO5, which leads to its augmented nuclear retention. Ectopic expression of a constitutively active form of MEK, i.e., MEKDD, restored XPO5’s nuclear localization in METTL1-deficient cells, suggesting that METTL1 modulates miRNA processing through a mechanism independent of its catalytic activity. Together, we uncovered a novel role of METTL1 in modulating XPO5’s subcellular distribution and pre-miRNA export through its interaction with XPO5, and we also revealed a novel mechanism of miRNA maturation and expanded METTL1’s function beyond m7G methylation.