Mapping the type I collagen-binding site on pigment epithelium-derived factor. Implications for its antiangiogenic activity

Pigment epithelium-derived factor (PEDF), a neurotrophic and antiangiogenic serpin, is identified in tissues rich in collagen, e.g. cornea, vitreous, bone, and cartilage. We show that recombinant human PEDF formed complexes with collagens from the bovine cornea and vitreous. We have examined the direct binding of PEDF to collagen I and found that interactions were ionic in nature and occurred when PEDF and collagen I were both in solution, when either one was immobilized, or even when collagen I was denatured under reducing conditions. (125)I-PEDF bound to immobilized collagen I in a saturable fashion (K(D) = 123 nm). Compared with neurotrophic PEDF-derived peptides, ovalbumin and angiogenic inhibitors, only full-length PEDF competed efficiently with (125)I-PEDF for the binding to immobilized collagen I (EC(50) = 3 microg/ml). The collagen-binding region was analyzed using controlled proteolysis and chemically modified PEDF. Cleavage of the serpin exposed loop did not prevent binding to collagen I. Conjugation of lysines with fluorescein increased the collagen binding affinity. However, treatment of PEDF with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide abolished it, implicating the PEDF aspartic and/or glutamic acid residues in its interaction with collagen I. A negatively charged region on the surface of the PEDF molecule is rich in acidic residues (Glu(41), Glu(42), Glu(43), Asp(44), Asp(64), Asp(256), Asp(258), Glu(290), Glu(291), Glu(296), Asp(300), Glu(304)) available to interact directly with positively charged areas of collagen. This represents the first collagen-binding site described for a serpin, which in PEDF, is distinct from its heparin-binding region, neurotrophic active site, and its serpin exposed loop. The collagen-binding property of PEDF may play a role in surface localization and modulation of its antiangiogenic effects in the eye and bone.