Divalent cation modulation of fibronectin binding to heparin and to DNA.

Fibronectin is an adhesive glycoprotein that binds to heparin and to DNA. The binding of tryptic fragments of human plasma fibronectin to these ligands is found to be highly dependent on the concentration of divalent cations. We have identified 3 types of binding to heparin. 1) Calcium-sensitive binding is inhibited by CaCl2, but not by MgCl2 or by MnCl2. The NH2-terminal 31,000-dalton fragment (fragment 23) has this type of binding, which is half-maximally inhibited by 3 to 4 mM CaCl2. 2) Divalent cation-sensitive binding is exhibited by a 75,000-dalton fragment (fragment 13); its binding is inhibited by all 3 divalent cations. 3) Divalent cation-insensitive binding is characteristic of a 95,000-dalton fragment (fragment 10) and larger fragments. These 3 fragments (fragments 10, 13, and 23) are not disulfide-bonded to other fragments. Specific tryptic fragments of fibronectin also bind readily to native DNA in the presence of EDTA, but the binding of all fragments is abolished by the presence of 10 mM CaCl2 or MgCl2. Our results indicate that the binding of specific domains of fibronectin to heparin or to DNA can be modulated by divalent cations.