Figure 1. METTL21A-mediated methylation of HSPA1-K561.

(a) METTL21A knockout abrogates methylation of HSPA1-K561 in vivo. Top panel, extracted ion chromatograms corresponding to mass-to-charge ratios of the various methylated forms (Kme0, Kme1, Kme2 and Kme3) of a previously studied²,⁴ AspN-generated proteolytic peptide covering Asp555–Ala565 of HSPA1 from KBM-7 wild-type (WT) and corresponding METTL21A knock-out (M21A-KO) cells. Bottom panel, western blot analysis of METTL21A and SETD1A in cell lines analysed in the upper panel. β-Actin was used as a loading control. (b) METTL21A-mediated methylation of HSPA1-K561 is non-processive. Recombinant human HSPA1 was treated with varying amounts of METTL21A in the presence of the methyl donor S-adenosyl methionine and the relative abundance of the various methylated forms of K561 was determined by mass spectrometry.