A receptor-like protein-tyrosine phosphatase PTPζ/RPTPβ binds a heparin-binding growth factor midkine -: Involvement of arginine 78 of midkine in the high affinity binding to PTPζ

Midkine is a 13-kDa heparin-binding growth factor with 45% sequence identity to pleiotrophin. Pleiotrophin has been demonstrated to bind to protein-tyrosine phosphatase zeta (PTP zeta) with high affinity. In this study, we examined the binding of midkine to PTP zeta by solid-phase binding assay. Midkine and pleiotrophin binding to PTP zeta were equally inhibited by soluble pleiotrophin and also by some specific glycosaminoglycans. For both bindings, Scatchard analysis revealed low (3.0 nM) and high (0.58 nM) affinity binding sites. These results suggested that PTP zeta is a common receptor for midkine and pleiotrophin. Midkine is structurally divided into the N- and C-terminal halves, and the latter exhibited full activity for PTP zeta binding and neuronal migration induction. The C-terminal half contains two heparin-binding sites consisting of clusters of basic amino acids, Clusters I and II. A mutation at Arg(78) in Cluster I resulted in loss of the high affinity binding and reduced neuronal migration-inducing activity, while mutations at Lys(83) and Lys(84) in Cluster II showed almost no effect on either activity. Chondroitinase ABC-treated PTP zeta exhibited similar low affinity binding both to the native midkine and midkine mutants at Arg(78). These results suggested that Arg(78) in midkine plays an essential role in high affinity binding to PTP zeta by interacting with the chondroitin sulfate portion of this receptor.