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, neuroca n and phosphacan (the extracellular domain of protein-tyrosine phospha tase zeta/P), 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 dele tion variants except those lacking the fibrinogen-like globe and that phosphacan binds to this single domain with nearly the same affinity ( K-d similar to 12 nM) as to native or recombinant tenascin-C. However, maximum binding of neurocan requires both the fibrinogen globe and so me of the adjacent fibronectin type III repeats. Binding of phosphacan and neurocan to intact tenascin-C, and of phosphacan to the fibrinoge n globe, is significantly increased in the presence of calcium. Chondr oitinase treatment of the proteoglycans did not affect their binding t o either native tenascin-C or to any of the recombinant proteins, demo nstrating that these interactions are: mediated by the proteoglycan co re proteins rather than through the glycosaminoglycan chains. These re sults are also consistent with rotary shadowing electron micrographs t hat show phosphacan as a rod terminated at one end by a globular domai n 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 f actor-like domains or the fibronectin type III repeats and the fibrino gen globe, In both cases cell adhesion was inhibited by similar concen trations of phosphacan, demonstrating that the fibrinogen globe is not necessary for this effect, which is apparently mediated by a direct a ction of phosphacan on the cells rather than by its interaction with t he proteoglycan binding site on tenascin-C.