Abstract
Transforming growth factor- g (TGF- g) is a multifunctional growth and differentiation factor that affects almost all cells. Although equipotent in many cases, the three isoforms of TGF- g (- g 1, - g 2, - g 3) have several important isoform specific activities. For example, TGF- g 2 binds with higher affinity to a 60 v kDa cell-surface glycosyl phosphatidylinositol (GPI)-linked protein, expressed on vascular endothelial cells. We used chimeric TGF- g proteins, in which selected regions of TGF- g 1 had been exchanged for the corresponding region of TGF- g 2, to demonstrate that amino acids 67 and 68 regulate binding of TGF- g to this protein. Exchange of amino acids 67 and 68 of TGF- g 1 into TGF- g 2 resulted in a protein similar in affinity to TGF- g 1 for binding to the GPI-linked protein. In contrast, exchange of only amino acid 67 of TGF- g 1 into TGF- g 2, or exchange of only amino acid 68 of TGF- g 1 into TGF- g 2, resulted in a protein with affinity similar to that of TGF- g 2. This suggests that the coordinated change of Gln and His of TGF- g 1 to Thr and Ile at positions 67 and 68 alters the specificity of TGF- g. Amino acids 67 and 68 are part of a surface-exposed f -helix that forms a projection away from the center of the TGF- g molecule and is accessible for receptor binding.