Histone methyltransferase that specifically monomethylates Lys-4 of histone H3. H3 Lys-4 methylation represents a specific tag for epigenetic transcriptional activation. Plays a central role in the transcriptional activation of genes such as collagenase or insulin. Recruited by IPF1/PDX-1 to the insulin promoter, leading to activate transcription. Has also methyltransferase activity toward non-histone proteins such as p53/TP53, TAF10, and possibly TAF7 by recognizing and binding the [KR]-[STA]-K in substrate proteins. Monomethylates Lys-189 of TAF10, leading to increase the affinity of TAF10 for RNA polymerase II. Monomethylates Lys-372 of p53/TP53, stabilizing p53/TP53 and increasing p53/TP53-mediated transcriptional activation.