AbstractIn a previous study, we have shown that the gene promoter of a protein termed KIAA0082 is regulated by interferon and that this protein interacts with the RNA polymerase II. It has been subsequently shown that KIAA0082 is the human cap‐specific messenger RNA (mRNA) (nucleoside‐2′‐O‐)‐methyltransferase 1 (hMTr1), which catalyzes methylation of the 2′‐O ‐ribose of the first nucleotide of capped mRNAs. Pre‐mRNAs are cotranscriptionally processed, requiring coordinate interactions or dissociations of hundreds of proteins. hMTr1 potentially binds to the 5′‐end of the whole cellular pre‐mRNA pool. Besides, it contains a WW protein interaction domain and thus is expected to be associated with several proteins. In this current study, we determined the composition of complexes isolated by hMTr1 immunoprecipitation from HEK293 cellular extracts. Consistently, a large set of proteins that function in pre‐mRNA maturation was identified, including splicing factors, spliceosome‐associated proteins, RNA helicases, heterogeneous nuclear ribonucleoproteins (HNRNPs), RNA‐binding proteins and proteins involved in mRNA 5′‐ and 3′‐end processing, forming an extensive interaction network. In total, 137 proteins were identified in two independent experiments, and some of them were validated by immunoblotting and immunofluorescence. Besides, we further characterized the nature of several hMTr1 interactions, showing that some are RNA dependent, including PARP1, ILF2, XRCC6, eIF2α, and NCL, and others are RNA independent, including FXR1, NPM1, PPM1B, and PRMT5. The data presented here are consistent with the important role played by hMTr1 in pre‐mRNA synthesis.