Takashi Muramatsu

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

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 ζ (PTPζ) with high affinity. In this study, we examined the binding of midkine to PTPζ by solid-phase binding assay. Midkine and pleiotrophin binding to PTPζ 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 PTPζ 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 PTPζ 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 Arg78 in Cluster I resulted in loss of the high affinity binding and reduced neuronal migration-inducing activity, while mutations at Lys83 and Lys84 in Cluster II showed almost no effect on either activity. Chondroitinase ABC-treated PTPζ exhibited similar low affinity binding both to the native midkine and midkine mutants at Arg78. These results suggested that Arg78 in midkine plays an essential role in high affinity binding to PTPζ by interacting with the chondroitin sulfate portion of this receptor.