Component of iboga and aspidosperma monoterpenoid indole alkaloids (MIAs, e.g. tabersonine and catharanthine) biosynthesis pathway from 19E-geissoschizine. Acetyltransferase that catalyzes the formation of O-acetylstemmadenine from stemmadenine. 15alpha-stemmadenine + acetyl-CoA = CoA + O-acetyl-15alpha-stemmadenine Alkaloid biosynthesis. Monomer. Expressed in leaf epidermis. Abnormal presence of stemmadenine, but normal accumulation of catharanthine and vindoline. Belongs to the plant acyltransferase family.