Structure and expression of the membrane proteoglycan betaglycan, a component of the TGF-β receptor system

We describe the primary structure of rat betaglycan, a polymorphic membrane-anchored proteoglycan with high affinity for transforming growth factor-β (TGF-β). As deduced from its cDNA sequence, the 853 amino acid core protein of betaglycan has an extracellular domain with clustered sites for potential attachment of glycosaminoglycan chains. These chains are dispensable for TGF-β binding to the core protein. The transmembrane region and the short cytoplasmic tail of betaglycan are very similar to these regions in human endoglin, an endothelial cell membrane glycoprotein involved in intercellular recognition. The ectodomain of betaglycan can be released as a soluble proteoglycan; a potential cleavage site near the transmembrane region is identical to the highly regulated cleavage site of the membrane-anchored transforming growth factor-α precursor. The unique features of betaglycan suggest important roles in cell interaction with TGF-β.