To identify METTL-interacting proteins, the total eluates from each purification were processed for peptide identification by liquid chromatography-tandem mass spectrometry (LC-MS). As expected, peptide sequences corresponding to METTL2A, METTL2B and METTL6 were detected in the respective Strep-METTL2A, 2B and 6 purifications (Fig. 1b). In addition, both the METTL2A and 2B purifications contained peptides corresponding to an uncharacterized protein encoded by the DALR anticodon-binding protein 3 (DALRD3) gene (Fig. 1b, Supplementary Data 1). The molecular weights of the two predicted DALRD3 isoforms are 55 and 59 kDa, which correspond in size to the unidentified bands within the doublet observed by silver stain. For the METTL6 purification, we detected numerous peptides for seryl-tRNA synthetase (SARS) (Fig. 1b, Supplementary Data 1). The molecular weight of SARS (59 kDa) matches closely in size to the 60 kDa band detected by silver stain in the METTL6 purification. Moreover, SARS was recently identified as an interactor of METTL632. However, no peptides corresponding to DARLD3 were identified in the METTL6 purification. These results suggest that DALRD3 forms a unique interaction with METTL2A and 2B.
