Substrate specificity of MLL3 protein variants. H3 (1 to 15) peptide arrays containing H3K4 at different methylation states and a K4A peptide were methylated with MLL3-SET wild-type and mutant proteins in the presence of complex members using radioactively labeled AdoMet. (A) Example of an autoradiographic image of the methylated peptide SPOT arrays, peptides with the corresponding lysine variants are indicated. (B) Quantitative analysis of the average methylation signals of two independent experiments. The methylation data were normalized to the H3K4me0 methylation signal obtained with the individual MLL3-SET variants. The error bars indicate the standard error of the mean.