Protein tyrosine phosphatase PTP-S binds to the juxtamembrane region of the hepatocyte growth factor receptor Met

Abstract

We reported previously that a protein tyrosine phosphatase (PTP) activity is associated with the immunoprecipitated hepatocyte growth factor (HGF) receptor, also known as Met. The activity increased reversibly when Met was stimulated by HGF and decreased when Met was inactivated by PMA. To identify the PTP-binding region, we used deletion mutants of the receptor beta-subunit. The PTP activity did not associate with Tpr-Met, a construct containing residues 1010-1390 of Met fused to Tpr. In contrast, PTP activity was present when the expressed protein contained the full juxtamembrane region (residues 956-1390 of Met) or part of this region (residues 957-1390 or 995-1390), indicating that the PTP-binding region is between residues 995 and 1009. This region includes Tyr1003, a site involved in Met downstream signalling. Incubation of Met immunoprecipitated from GTL-16 cells with an 8-mer phosphopeptide derived from residues 1003-1010 induced a marked decrease in the associated PTP activity, suggesting that the peptide reproduced the PTP-binding region. Mutation of Glu, Asp or Arg at positions -4, -1 or +1 respectively relative to Tyr1003 in a 9-mer peptide (residues 999-1007) abolished the ability of the peptide to decrease the PTP activity associated with Met. Phosphorylation of Tyr1003 was not required for PTP binding, since: (1) both phospho- and dephospho-peptides on a solid bead bound PTP activity from a GTL-16 cell extract, and (2) PTP activity was associated with a Met deletion mutant lacking residues 1-955 in which Tyr1003 had been changed into Phe. In order to partially purify the PTP from the GTL-16 cell extract, an affinity column was prepared using the Met-derived peptide comprising residues 998-1007. Less than 0.1% of the total cellular PTP was retained by the column, and was eluted with low salt concentrations. Using antibodies, this PTP was identified as PTP-S, a soluble PTP present in epithelial cells and fibroblasts.