title: Evidence that binding to the carboxyl-terminal heparin-binding domain (Hep II) dominates the interaction between plasma fibronectin and heparin.
abstract: We assessed the participation of the three known heparin-binding domains of PFn (Hep I, Hep II, Hep III) in their interaction with heparin by making a quantitative comparison of the fluid-phase heparin affinities of PFn and PFn fragments under physiologic pH and ionic strength conditions. Using a fluorescence polarization binding assay that employed a PFn affinity-purified fluorescein-labeled heparin preparation, we found that greater than 98% of the total PFn heparin-binding sites exhibit a Kd in the 118-217 nM range. We also identified a minor (less than 2%) class of binding sites exhibiting very high affinity (Kd approximately 1 nM) in PFn and the carboxyl-terminal 190/170 and 150/136 kDa PFn fragments. This latter activity probably reflects multivalent inter- or intramolecular heparin-binding activity. Amino-terminal PFn fragments containing Hep I (72 and 29 kDa) exhibited low affinity for heparin under physiologic buffer conditions (Kd approximately 30,000 mM). PFn fragments (190/170 and 150/136 kDa) containing both the carboxyl-terminal Hep II and central Hep III domains retained most of the heparin-binding activity of native PFn (Kd = 278-492 nM). The isolated Hep II domain (33-kDa fragment) exhibited appreciable, but somewhat lower (2-5-fold), heparin affinity compared to the 190/170-kDa PFn fragment. Heparin binding to the 100-kDa PFn fragment containing Hep III was barely detectable (Kd greater than 30,000 nM). From these observations, we conclude that PFn contains only one major functional heparin-binding site per subunit, Hep II, that dominates the interaction between heparin and PFn.
authors.affiliation: Department of Medicine, University of Wisconsin Medical School, Campus, Milwaukee 53233.
authors.forename: M J
authors.initials: MJ
authors.lastname: Benecky
authors.forename: C G
authors.initials: CG
authors.lastname: Kolvenbach
authors.forename: D L
authors.initials: DL
authors.lastname: Amrani
authors.forename: M W
authors.initials: MW
authors.lastname: Mosesson
mesh_terms: D001665:Binding Sites; D005353:Fibronectins / Q000378:metabolism*; D005454:Fluorescence Polarization; D006493:Heparin / Q000378:metabolism*; D006801:Humans; D008970:Molecular Weight; D010446:Peptide Fragments / Q000378:metabolism
chemical_list: D005353:Fibronectins; D010446:Peptide Fragments; D006493:Heparin