Catalyzes the methylation of the lipid moiety of the intermediate compounds phthiotriol and glycosylated phenolphthiotriol dimycoserosates to form phthiocerol dimycocerosates (DIM A) and glycosylated phenolphthiocerol dimycocerosates (PGL). Was identified as a high-confidence drug target. Belongs to the methyltransferase superfamily. Phthiotriol/phenolphthiotriol dimycocerosates methyltransferase family.