The PTPμ Protein-tyrosine Phosphatase Binds and Recruits the Scaffolding Protein RACK1 to Cell-Cell Contacts

Abstract

PTPμ, an Ig superfamily receptor protein-tyrosine phosphatase, promotes cell-cell adhesion and interacts with the cadherin-catenin complex. The signaling pathway downstream of PTPμ is unknown; therefore, we used a yeast two-hybrid screen to identify additional PTPμ interacting proteins. The membrane-proximal catalytic domain of PTPμ was used as bait. Sequencing of two positive clones identified the scaffolding protein RACK1 (receptor for activated protein C kinase) as a PTPμ interacting protein. We demonstrate that RACK1 interacts with PTPμ when co-expressed in a recombinant baculovirus expression system. RACK1 is known to bind to the src protein-tyrosine kinase. This study demonstrates that PTPμ association with RACK1 is disrupted by the presence of constituitively active src. RACK1 is thought to be a scaffolding protein that recruits proteins to the plasma membrane via an unknown mechanism. We have shown that the association of endogenous PTPμ and RACK1 in a lung cell line is increased at high cell density. We also demonstrate that the recruitment of RACK1 to both the plasma membrane and cell-cell contact sites is dependent upon the presence of the PTPμ protein in these cells. Therefore, PTPμ may be one of the proteins that recruits RACK1 to points of cell-cell contact, which may be important for PTPμ-dependent signaling in response to cell-cell adhesion.