Transforming growth factor beta isoform 2-specific high affinity binding to native alpha 2-macroglobulin. Chimeras identify a sequence that determines affinity for native but not activated alpha 2-macroglobulin.

Abstract

Transforming growth factor beta 2 (TGF-beta 2) is less potent than TGF-beta 1 in some endothelial cell proliferation assays due to the greater tendency of TGF-beta 2 to bind alpha 2-macroglobulin (alpha 2M). Substitution of TGF-beta 1 residues 40-47 into the TGF-beta 2 sequence yields a chimeric molecule that, like TGF-beta 1, expresses activity that is not substantially affected by serum alpha 2M (Burmester, J. K., Qian, S. W., Roberts, A. B., Huang, A., Amatayakul-Chantler, S., Suardet, L., Odartchenko, N., Madri, J. A., and Sporn, M. B. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 8628-8632). In this investigation, we studied the binding of TGF-beta chimeras, which contain TGF-beta 1 residues 40-47, to both major conformations of human alpha 2M under apparent equilibrium conditions. Native alpha 2M, the primary form of this protein in serum, bound TGF-beta 2/beta 1 (40-82) and TGF-beta 2/beta 1 (40-47) with low affinity. The apparent KD values for the two chimeras and native alpha 2M were 310 and 330 nM, respectively. These values were much higher than the KD determined for TGF-beta 2 and native alpha 2M (11 nM) and equivalent to the KD determined for TGF-beta 1 and native alpha 2M. By contrast, both TGF-beta chimeras bound alpha 2M-methylamine, an altered conformation of alpha 2M, with high affinity (16 and 19 nM), which is characteristic of TGF-beta 2 and not TGF-beta 1. Fetal bovine heart endothelial cell DNA synthesis was inhibited to a similar degree by TGF-beta 1, TGF-beta 2, TGF-beta 2/beta 1 (40-82), and TGF-beta 2/beta 1 (40-47) in the presence of dilute (0.2%) fetal bovine serum. When 0.07 microM alpha 2M-methylamine was added, the activities of TGF-beta 2, TGF-beta 2/beta 1 (40-82), and TGF-beta 2/beta 1 (40-47) were significantly counteracted while the activity of TGF-beta 1 was unchanged, as would be predicted by the equilibrium binding analyses. These studies indicate that the TGF-beta structural elements, which mediate binding to native alpha 2M and conformationally transformed alpha 2M, are not equivalent. Residues 40-47 are critical in determining affinity for native alpha 2M but are less important in determining affinity for alpha 2M-methylamine.