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. Widely expressed with high level in uterus, aorta, heart, lung, trachea, placenta and in fetal heart, kidney, liver, spleen and lung. Expressed by CD34(+) acute myeloid leukemia cell lines, T-cell lines, activated T-lymphoblasts, endothelial cells and activated platelets. Isoform 4 is expressed in placenta. Isoform 1 is expressed in keratinocytes and placenta. N-glycosylated. 2 forms of 150 (p150) and 120 kDa (p120) exist due to proteolytic degradation from a 180 kDa form. The Gov(b) variant in position 703 defines the Gov alloantigenic determinants. Belongs to the protease inhibitor I39 (alpha-2-macroglobulin) family. Truncated N-terminus. Extended N-terminus.