| The reversible and dynamic tyrosine phosphorylation is governed by opposing activities of protein tyrosine kinases (PTKs) and protein tyrosine phosphatase (PTPs), which is essential for regulating cellular growth, differentiation, metabolism, cell cycle, cell-cell communication, cell migration, gene transcription, ion-channel activity, immune response and survival. Because many PTKs are oncogenes and growth factors, most studies on tyrosine phosphorylation are focused on PTKs. However, the importance of PTPs has not been recognized until recent ten years. Structurely, PTPs have been found in transmembrane receptor-like, and soluble cytosolic forms. The subfamily of receptor-like PTPs (RPTPs) is composed of seven classes and typeII and III RPTPs are characterized by their extracellular immunoglobin-like domain and fibronectin-III repeats, which resembles those of cell adhesion molecules and implicates that they may be involved in the regulation of cell adhesion. In fact, PTPμ and PTPκ have been proved to induce cell aggregation by homophilic interaction of their respective extracellular regions.PCP-2, a human receptor-like protein tyrosine phosphatase with a transcript of 5581 base pairs in length, was identified by Wang HY et al in human pancreatic adenocarcinoma cells in 1996. It shows high homolog to PTPμ and PTPκ and encodes a protein of 1430 amino acids with the activity of phosphatase. PCP-2 is co-localized with E-cadherin and (-catenin at cell junction. The intracellular segment of PCP-2 has two phosphatase domains and a relatively long juxtamembrane segment that ishomologous to the conserved cytoplasmic domain of cadherins, which is essential for the complex formation with the intracellular catenins. And catenins in turn link cadherins to the actin filament net work. Cadherins may thus be involved in the regulation of adhesion. There is a growing body of evidences to implicate reversible tyrosine phosphorylation as an important mechanism in the control of the adhesion function by cadherins/catenins complex. So far, there have been few reports on PCP-2 and this is the first report on the interaction of PCP-2 and catenins and its effect on cell adhesion and migration.In this report, we aimed to investigate the interaction between PCP-2 and β-catenin and its effect on cell adhesion and migration by the widely recognized methods such as Western Blot, Northern Blot, co-immunoprecipitation, point mutation, deletion mutation and transient or stable gene transfection. In WRL68 and COS-7 cells, we unexpectedly detected only one specific protein band and failed to detect any cleavage products both in endogenous and ecotopic expression by Western Blot and immunoprecipitation, which was greatly different with PTPμ and PTPκ. We showed here that the expression levels of both PCP-2 mRNA and protein were positively dependent on cell density. The mRNA level of WRL68 cells with 100 % confluence was three times higher than that of 30 % confluence and the protein level was elevated to 5.5 times, which might result from the regulation of both transcription and post-transcription level and revealed that PCP-2 might regulate cell-cell contact. We expressed and purified Histidine-tagged β-catenin carboxyl terminal (CT) fusion protein, and obtained anti-β-catenin CT polyclonal antibody with a dilution of 1:500 by immunizing NewZealand white rabbits with the fusion protein. Using in vivo binding assay, we demonstrated specific complex formation between PCP-2 and β-catenin and the association was independent of the tyrosine phosphorylation state of β-catenin. We obtained three deletion mutants and mutation analysis revealed that the juxtamembrane region of PCP-2 was sufficient for binding with β-catenin. In addition, we also obtained two point mutants, PCP-2 C1069S and PCP-2 C1364S, and usingco-transfection experiments with activated SrcY527F, β-catenin and PCP-2 or inactive PCP-2, we presented evidence that β-catenin might be a substrate for the catalytic activity of PCP-2 and c...
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