Binding of hexabrachions to heparin and DNA

Hexabrachions are extracellular proteins expressed in certain tissues and at specific points in development. cDNA sequencing has revealed that they contain a region of repeats that are similar to the type III homology units of fibronectin. The corresponding region of fibronectin contains heparin- and DNA-binding domains. We have compared the heparin and DNA binding of hexabrachion secreted by the human glioblastoma cell line U87MG to that of fibronectin. Both proteins bound to heparin-agarose in low salt (0.05 M NaCl) buffers. Using linear salt gradients, hexabrachion was eluted from heparin prior to fibronectin. The addition of 5 mM CaCl2 decreased the affinity of both proteins for heparin, but it had a greater effect upon the binding of fibronectin. Free heparin but not chondroitin sulfate inhibited the binding of both proteins to heparin-agarose. In addition, hexabrachion bound to DNA as fibronectin does, and this binding could be inhibited by heparin but not by chondroitin sulfate. Unlike fibronectin, hexabrachion did not bind to gelatin when samples containing both proteins were passed over gelatin-agarose, also indicating that there was no interaction between hexabrachion and fibronectin. In contrast to hexabrachion isolated from brain, the protein secreted by the human glioblastoma cell line U87MG does not bear the HNK-1 epitope which is on a carbohydrate that can mediate interactions between cells.