The in vitro interaction of proteoglycans with type I collagen is modulated by phosphate

Binding of proteoglycans to type I collagen in vitro was assessed using radiolabeled decorin, biglycan, and large proteoglycans and acid-extracted bovine tendon collagen. Decorin, biglycan, and large proteoglycans were all bound to collagen fibrils in phosphate-buffered saline (PBS) containing 3 mM sodium phosphate. Only decorin was bound when the phosphate concentration in PBS was increased to 30 mM. These distinct binding characteristics were not altered by the presence of 10% serum, by purification of the proteoglycans in 7 M urea and 4 M guanidine HCl, or by digestion of the collagen with pepsin. In addition to being affected by phosphate, both glycosaminoglycan and proteoglycan binding to collagen was inhibited by sulfate, an anion with similar structure, and by molecules that contain sulfate or sulfonate groups, such as chondroitin sulfate and N-tris[hydroxymethyl]methyl-2-aminoethanesulfonic acid (Tes). The rate of in vitro collagen fibrillogenesis was retarded by increasing concentrations of phosphate. Decorin decreased the rate of collagen fibrillogenesis in all buffers and virtually abolished fibril formation when added in buffer containing both 30 mM phosphate and 30 mM Tes. It is concluded that decorin binds to collagen through interaction between collagen and the decorin core protein, whereas biglycan and large proteoglycans bind to collagen fibrils through their glycosaminoglycan chains. This glycosaminoglycan-collagen interaction is inhibited by phosphate, sulfate, and sulfonate. These observations may clarify contradictory results among previous in vitro studies of proteoglycan-collagen interaction. Since the phosphate concentration of blood and interstitial fluid is estimated to be approximately 1 mM, collagen-glycosaminoglycan interactions could occur in tissue.