Biochemical analysis of the interaction of fibronectin with IgG and localization of the respective binding sites

Abstract

Fibronectin (Fn), a mosaic protein composed of multiple copies of three different module types (F1, F2 and F3), has been found associated with circulating immune complexes (ICs) and immunoglobulin (Ig) aggregates in a variety of IC diseases and myeloproliferative disorders. We have previously shown that a proteolytic fragment of M r = 25,900 Da, from the NH 2-terminal domain of Fn, composed of five type 1 modules ( 1F1- 5F1) binds to the major Ig classes under physiologic conditions, suggesting that the presence of Fn in ICs and cryoglobulins results from a physicochemical binding interaction between these two molecules. Using an ELISA, we now show that the interaction between Fn and IgG is: (1) not influenced by any other constituent of plasma; (2) unaffected by temperature; and (3) has an estimated K d of 3.77 × 10 −9 M. In addition, we have further delineated the respective sites involved in the interaction between Fn and IgG. Recombinant type 1 module pairs ( 1F1. 2F1 and 4F1. 5F1) from the NH 2-terminus of Fn, expressed in yeast, were employed in an ELISA and affinity chromatography and compared with the 25.9 kDa ( 1F1- 5F1) fragment and intact Fn for binding to IgG. The 4F1. 5F1 and the 25.9 kDa fragment bound to immobilized IgG and inhibited Fn binding to IgG to nearly the same extent as the intact molecule (IC 50: Fn = 6.77 × 10 −9 M; 25.9 kDa fragment = 5 × 10 −9 M; 4F1. 5F1 = 7.6 × 10 −9 M). Thus, the binding site for IgG on the Fn molecule is localized to and completely conferred by the 4F1. 5F1 module pair (residues 151–244). Similar experiments using papain-generated Fab and Fc fragments of IgG localized the Fn binding site on IgG to the Fc region of the IgG molecule. Fn bound to the Fc fragment with a nearly identical K d of 3.69 × 10 −9 M, as to intact IgG (3.77 × 10 −9 M). These studies support the hypothesis that the interaction between Fn and Ig may contribute to the pathophysiology of immune complex related disorders.