Decorin Core Protein Fragment Leu155-Val260 Interacts with TGF-β but Does Not Compete for Decorin Binding to Type I Collagen

It has been shown that small proteoglycans containing leucine-rich repeats in their core proteins can form complexes with TGF-β. Decorin, a ubiquitously found molecule of the extracellular matrix, is the best-studied example. Therefore, binding domains on its core protein were investigated using recombinant decorin fragments generated as fusion proteins in prokaryotes. The peptide Leu155-Val260 immobilized by the polyhistidine tag on a nickel chelate column bound TGF-β1 and -β2 almost as effectively as the largest fragment (Asp45-Lys359) studied. Other peptides were less effective. For the two peptides Asp45-Lys359 and Leu155-Val260 dissociation constants in the nanomolar range for high-affinity binding sites were calculated in a solid-phase assay with immobilized TGF-β2. Peptide Asp45-Lys359 also contained a lower affinity binding site. Domains with lower affinity were also found in peptides Asp45-Leu155 and Arg63-Gly190. Peptide Leu155-Val260 also formed complexes with TGF-β in the liquid phase as determined by equilibrium gel filtration. Furthermore, F(ab′) fragments of polyclonal antibodies against peptide Leu155-Val260 interfered with TGF-β binding to peptide Asp45-Lys359 in a dose-dependent manner. Peptide Leu155-Val260, however, is only a weak competitor of the binding of wild-type decorin to reconstituted type I collagen fibrils. Therefore, independent binding sites of decorin for TGF-β and type I collagen should exist. In support of this hypothesis saturable binding of TGF-β1 and TGF-β2 to collagen-bound native decorin could be demonstrated. The bound cytokine could be released in a biologically active form by collagenase treatment. Thus, decorin may play a biological role in storing this cytokine temporarily in the extracellular matrix and in thereby modulating an interaction of TGF-β with its signaling receptors.