Modulates negatively TGFB1 signaling in keratinocytes. Heterodimer; disulfide-linked. Interacts with TGFB1 and TGFBR1. Forms a heteromeric complex with TGFBR1, TGFBR2 and TGFBR3 in a ligand-independent manner (By similarity). N-glycosylated. 2 forms of 150 (p150) and 120 kDa (p120) exist due to proteolytic degradation from a 180 kDa form. Belongs to the protease inhibitor I39 (alpha-2-macroglobulin) family.