The endocytic system is a series of compartments that determine the fate of intracellular protein cargo. Such cargo is commonly delivered from the plasma membrane to endosomes where trafficking decisions are made. Endocytic proteins may also serve as a platform for protein-protein interactions to regulate a range of normal cellular processes such as migration, polarity and cytokinesis. Alterations in the endocytosis and recycling of plasma membrane proteins are observed in some diseases including cystic fibrosis and polycystic kidney disease. Thus, identifying critical proteins that regulate the movement of cellular proteins through endosomes has the potential to increase our understanding of the cell biology underlying many disease processes. Out lab currently studies the role of the early endosomal sorting nexin (SNX), SNX27. Our lab first identified SNX27 in a proteomic screen for proteins that bind the C-terminus of the cystic fibrosis chloride channel CFTR. We recently complete a functional study of the CFTR-SNX27 interaction;however because SNX27 is very broadly expressed while CFTR is restricted to a small set of exocrine and epithelial tissues, we wished to expand our efforts beyond CFTR. Thus the focus of our new work has been to identify and characterize novel SNX27-interacting proteins. We used mass spectrometry to identify proteins that co-immunoprecipitate with HA-SNX27 and to identify proteins that bind various GST-SNX fusion proteins in pull-down assays. Thus far, we have identified and confirmed several interacting proteins, including the tight junction protein ZO-2, the adherens junction protein β-catenin, and β-Pix, a guanine nucleotide exchange factor. We also identified Git-1 and Git-2 in our screens;the SNX27-Git interaction is mediated by β-Pix. We are characterizing each of these novel protein interactions and have developed RNA interference strategies in mouse and human cells to knockdown SNX27 and study the functional implications. Our data and recently published data from other groups, suggests that SNX27 mediates the trafficking of multiple cell surface proteins via endosomes to multiple cellular compartments. On-going work in the lab will elucidate the function of each interaction and lay the groundwork for additional mechanistic studies.
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