BINDING OF HEPARIN BY TYPE-III DOMAINS AND PEPTIDES FROM THE CARBOXY-TERMINAL HEP-2 REGION OF FIBRONECTIN

The major sites of heparin binding by fibronectin are located in fragm ents of 30 or 40 kDa that contain type III modules 12 through 14 or 15 . Various proteolytic or recombinant subfragments and several syntheti c peptides derived from this region have been compared with respect to their binding to fluorescein-labeled heparin in solution. Binding was monitored by the change in fluorescence anisotropy at 25-degrees-C an d pH 7.4 in 0.02 M Tris buffer, alone (TB) or with 0.15 M NaCl (TBS). A 23-kDa fragment containing III13 and III14 but lacking III12 had K(d ) values of 0.3 and 1.8 muM in TB and TBS, respectively, indistinguish able from the 30-kDa parent. Fragments containing only module III13 bo und 2-3-fold weaker than the parent while those containing only III14 bound 6-50-fold weaker depending on the ionic strength. Fragments cont aining only III12 or III15 failed to bind at all in TBS. A cationic pe ptide derived from the amino terminus of III13 and containing the Arg- Arg-Ala-Arg consensus sequence, whose integrity was shown by Barkalow and Schwarzbauer [Barkalow, F. J., & Schwarzbauer, J. E. (1991) J. Bio l. Chem. 266, 7812-7818] to be critical, failed to bind in TBS but bou nd weakly in TB. Two additional cationic peptides derived from the mid dle and C-terminal regions of III14 showed similar behavior. Thus whil e the major determinant(s) of heparin binding are located in III13, th ose determinants are only active when part of a properly folded struct ure. Furthermore,module III13 when isolated had a slightly lower affin ity than fragments containing both III13 and III14. It is concluded th at interactions between these two modules may be important to arrange positively charged residues from both modules for optimal recognition by heparin.