Alpha N-methyltransferase that methylates the N-terminus of target proteins containing the N-terminal motif [Ala/Pro/Ser]-Pro-Lys when the initiator Met is cleaved. Specifically catalyzes monomethylation of exposed alpha-amino group of Ala or Ser residue in the [Ala/Ser]-Pro-Lys motif and Pro in the Pro-Pro-Lys motif (PubMed:24090352, PubMed:30417120). Predominantly functions as a mono- methyltransferase but is also able to di-/tri-methylate the GPKRIA peptide and di-methylate the PPKRIA peptide (in vitro) (PubMed:30417120). May activate NTMT1 by priming its substrates for trimethylation (PubMed:24090352). {ECO:0000269|PubMed:24090352, ECO:0000269|PubMed:30417120}. This is the function of NTMT2 (N-terminal Xaa-Pro-Lys N-methyltransferase 2, Ensembl gene identifier ENSG00000203740).