Involved in the biosynthesis of protoberberine alkaloids. N-methyltransferase with a substrate preference for (R,S)-norreticuline but also active with dimethoxytetrahydroisoquinoline. (S)-coclaurine + S-adenosyl-L-methionine = (S)-N-methylcoclaurine + H(+) + S-adenosyl-L-homocysteine Homodimer. Highly expressed in rhizomes. Detected in roots, petioles, flower buds and leaves. Expressed between the developing stele and ground tissues near the root apical meristem, in the immature endodermis, the pericycle and the spokes of developing xylem in the apical region of the root and in the protoderm of leaf primordia in rhizomes. Not induced by elicitor treatment. Belongs to the CFA/CMAS family.