An investigation into the mechanism of TMIGD1-mediated signal transduction pathway in human epithelial cells
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Dysregulation of protein expression, in particular expression of proto-oncogenes and tumor-suppressor genes whose function play key roles in cell growth, adhesion and migration, are hallmarks of human malignancies. Transmembrane and immunoglobulin-containing domain 1 (TMIGD1) was recently discovered as a cell adhesion molecule (CAM) that plays an important role in epithelial cell function by regulating epithelial cell polarity and adhesion. The extracellular domain of TMIGD1 contains two Ig domains that are involved in cell-cell interaction, followed by a transmembrane region and short cytoplasmic domain with potential to relay signal transduction. Our further investigation demonstrated TMIGD1 is downregulated in human colon cancer, suggesting a potentially important role for TMIGD1 in the regulation colorectal cancer. However, the molecular mechanisms of TMIGD1-mediated signal transduction, which could relay its function in epithelial cells, are not known. Using liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, we have identified moesin as a possible TMIGD1 binding protein. Moesin, a member of the Ezrin/Radixin/Moesin (ERM) family of proteins, is upregulated in human tumors. Moesin stimulates cell migration, tumor invasion, adherence and modulates cytoskeletal actin assembly. Similar to other ERM family proteins, moesin contains an N-terminal FERM domain, which binds to transmembrane proteins, and a C-terminal C-ERMAD domain, which binds F-actin. The overall goal of this study was to determine the binding of moesin with TMIGD1 and the specific domain involved in mediating the binding of moesin with TMIGD1. Our study in vitro and in vivo binding assays demonstrate that moesin interacts with the cytoplasmic domain of TMIGD1 via its FERM domain. Moreover, we demonstrate TMIGD1 interaction with moesin inhibits phosphorylation of moesin, indicating that perhaps TMIGD1 inhibits tumor cell migration through inhibition of phosphorylation of moesin. Additionally, TMIGD1 alters cellular localization of moesin, suggesting that altered cellular localization by TMIGD1 could account for inhibition of phosphorylation of moesin. We propose that TMIGD1 sequesters moesin near the cell membrane, preventing its interaction with PIP2, which is required for its phosphorylation and hence inhibits moesin activation. Altogether, the data presented in this work identifies moesin as a key signaling component of TMIGD1. Moesin directly interacts with TMIGD1 via its FERM domain. Recruitment of moesin to TMIGD1 blocks phosphorylation of moesin, suggesting that TMIGD1 exerts its effect in tumor cells in part by inhibition of moesin activation.