[Ser⁷⁷]transforming growth factor-β1: Selective biological activity and receptor binding in mink lung epithelial cells

Transforming growth factor-β1 (TGF-β1) is a homodimeric protein stabilized by a single disulfide bridge between Cys⁷⁷ on the respective monomers and two paired complementary hydrophobic interfaces between the two subunits. A TGF-β1 mutant with Cys⁷⁷ replaced by serine has been expressed in stably transfected Chinese hamster ovary cells and purified to homogeneity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis confirms that the sole interchain disulfide bond in TGF-β1 has been eliminated. It is 20% as potent as native TGF-β1 in the induction of plasminogen activator inhibitor-1 promoter expression in mink lung epithelial cells (Mv1Lu), although it is less than 1% as potent as native TGF-β1 in inhibition of growth in the same cell line. The mutant acts as a full agonist in both bioassays. [Ser⁷⁷]TGF-β1 binds to soluble type II receptors and competes with native TGF-β1 in sandwich-enzymelinked immunosorbent assays; however, in Mv1Lu cells, the mutant shows preferential cross-linking to type I rather than type II receptors. [Ser⁷⁷]TGF-β1 is a useful tool for understanding the different ligand-receptor complexes and numerous biological activities of this multifunctional cytokine.