Methyltransferase that mediates the second step of biosynthesis of ergot alkaloid biosynthesis. Catalyzes the N-methylation of 4-dimethylallyl-L-tryptophan in the presence of S-adenosyl-L-methionine, resulting in the formation of 4-dimethylallyl-L-abrine. Ergot alkaloids, which are produced by endophyte fungi, can enhance plant host fitness, but also cause livestock toxicosis to host plants. 4-(3-methylbut-2-enyl)-L-tryptophan + S-adenosyl-L-methionine = 4-(3-methylbut-2-enyl)-L-abrine + H(+) + S-adenosyl-L-homocysteine Alkaloid biosynthesis; ergot alkaloid biosynthesis. Homodimer. Belongs to the methyltransferase superfamily.