Transforming growth factor‐β1 is a heparin‐binding protein: Identification of putative heparin‐binding regions and isolation of heparins with varying affinity for TGF‐β1

Abstract

Previous studies indicated that a major factor in heparin's ability to suppress the proliferation of vascular smooth muscle cells is an interaction with transforming growth factor-β1 (TGF-β1). Heparin appeared to bind directly to TGF-β1 and to prevent the association of TGF-β1 with α2-macroglobulin (α2-M). The present studies indicate that 20–70% of iodinated TGF-β1 binds to heparin-Sepharose and the retained fraction is eluted with ∼0.37 M NaCI. Native, unlabelled platelet TGF-β1, however, is completely retained by heparin-Sepharose and eluted with 0.9–1.2 M NaCI. Using synthetic peptides, the regions of TGF-β1 that might be involved in the binding of heparin and other polyanions were examined. Sequence analysis of TGF-β1 indicated three regions with a high concentration of basic residues. Two of these regions had the basic residues arranged in a pattern homologous to reported consensus heparin-binding regions of other proteins. The third constituted a structurally novel pattern of basic residues. Synthetic peptides homologous to these three regions, but not to other regions of TGF-β1, were found to bind to heparin-Sepharose and were eluted with 0.15 M-0.30 M NaCI. Only two of these regions were capable of blocking the binding of heparin to ¹²⁵I-TGF-β. Immobilization of these peptides, followed by affinity purification of heparin, indicated that one peptide was capable of isolating subspecies of heparin with high and low affinity for authentic TGF-β1. The ability of TGF-β1 to bind to heparin or related proteoglycans under physiological conditions may be useful in understanding the biology of this pluripotent growth and metabolic signal. Conversely, a subspecies of heparin molecules with high affinity for TGF-β1 may be a factor in some of the diverse biological actions of heparin.