Methyltransferase involved in the biosynthesis of the benzylisoquinoline alkaloid noscapine (PubMed:22535422, PubMed:29723437, PubMed:27378283, PubMed:29610307). Catalyzes the conversion of (S)-scoulerine to (S)-tetrahydrocolumbamine (PubMed:22535422). The heterodimers SOMT2-SOMT3 and SOMT2-6OMT convert 3-O-acetyl-4'-O-demethylpapaveroxine to 3-O-acetylpapaveroxine, where SOMT2 is the catalytic subunit (PubMed:29723437, PubMed:27378283). (S)-scoulerine + S-adenosyl-L-methionine = (S)-tetrahydrocolumbamine + H(+) + S-adenosyl-L-homocysteine (S)-S-adenosyl-L-methionine + 3-O-acetyl-4'-O-demethylpapaveroxine = 3-O-acetylpapaveroxine + H(+) + S-adenosyl-L-homocysteine Alkaloid biosynthesis. Homodimer (PubMed:29723437). Forms heterodimers with SOMT3 and 6OMT (PubMed:29723437, PubMed:27378283). The heterodimers SOMT2-SOMT3 and SOMT2-6OMT possess 3-O-acetyl-4'-O-demethylpapaveroxine 4'-O-methyltransferase activity, where SOMT2 is the catalytic subunit (PubMed:29723437, PubMed:27378283). Homodimer does not possess 3-O-acetyl-4'-O-demethylpapaveroxine 4'-O-methyltransferase activity (PubMed:29723437). Highly expressed in capsules (PubMed:22653730). Expressed is stems (PubMed:22653730, PubMed:22535422). Expressed at low levels in roots (PubMed:22535422). A single amino acid substitution of Ser-122 in cultivar Bea's Choice (AC I3PLQ6) to Tyr-122 in cultivar Marianne (AC A0A2S1WBY6) abolishes the 3-O-acetyl-4'-O-demethylpapaveroxine 4'-O-methyltransferase activity. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.