METTL1 interacts with XPO5 to modulate pre-miRNA export.

While METTL1 is a well-established m7G writer protein, its protein-protein interaction network remains largely unexplored. To map the METTL1 interactome in HEK293T cells, we employed APEX2-mediated proximity labeling coupled with LC-MS/MS analysis. This approach allowed for the identification of 60 and 18 unique proteins significantly enriched in the METTL1 proximity proteome compared to enhanced green fluorescent protein (EGFP) and nuclear localization signal (NLS) controls, respectively. Among these proteins, we found exportin-5 (XPO5), a nuclear export factor critical for pre-miRNA transport. We validated the METTL1-XPO5 interaction by co-immunoprecipitation and western blot analysis. Strikingly, genetic ablation of METTL1 caused XPO5 to redistribute to the cytosol, which in turn accelerated pre-miRNA export and enhanced miRNA maturation. This function of METTL1 was independent of its canonical m7G methyltransferase activity. Mechanistically, we found that METTL1 facilitates ERK-mediated phosphorylation of XPO5, thereby promoting its nuclear retention. Accordingly, constitutive activation of ERK was sufficient to restore nuclear XPO5 localization in METTL1-deficient cells. In summary, our study uncovers a non-canonical role for METTL1 in regulating the subcellular distribution of XPO5 and pre-miRNA export, revealing a novel mechanism of miRNA maturation that extends METTL1's function beyond m7G methylation.