The fibrinogen-like globe of tenascin-C mediates its interactions with neurocan and phosphacan/protein-tyrosine phosphatase-zeta/beta

Two nervous tissue-specific chondroitin sulfate proteoglycans, neurocan and phosphacan (the extracellular domain of protein-tyrosine phosphatase-zeta/beta), are high-affinity ligands of tenascin-C. Using portions of tenascin-C expressed as recombinant proteins in human fibrosarcoma cells, we have demonstrated both by direct radioligand binding assays and inhibition studies that phosphacan binding is retained in all deletion variants except those lacking the fibrinogen-like globe and that phosphacan binds to this single domain with nearly the same affinity (Kd approximately 12 nM) as to native or recombinant tenascin-C. However, maximum binding of neurocan requires both the fibrinogen globe and some of the adjacent fibronectin type III repeats. Binding of phosphacan and neurocan to intact tenascin-C, and of phosphacan to the fibrinogen globe, is significantly increased in the presence of calcium. Chondroitinase treatment of the proteoglycans did not affect their binding to either native tenascin-C or to any of the recombinant proteins, demonstrating that these interactions are mediated by the proteoglycan core proteins rather than through the glycosaminoglycan chains. These results are also consistent with rotary shadowing electron micrographs that show phosphacan as a rod terminated at one end by a globular domain that is frequently seen apposed to the fibrinogen globe in mixtures of phosphacan and tenascin-C. C6 glioma cells adhere to and spread on deletion variants of tenascin-C containing only the epidermal growth factor-like domains or the fibronectin type III repeats and the fibrinogen globe. In both cases cell adhesion was inhibited by similar concentrations of phosphacan, demonstrating that the fibrinogen globe is not necessary for this effect, which is apparently mediated by a direct action of phosphacan on the cells rather than by its interaction with the proteoglycan binding site on tenascin-C.