A Membrane Fusion Protein αSNAP Is a Novel Regulator of Epithelial Apical Junctions

Tight junctions (TJs) and adherens junctions (AJs) are key determinants of the structure and permeability of epithelial barriers. Although exocytic delivery to the cell surface is crucial for junctional assembly, little is known about the mechanisms controlling TJ and AJ exocytosis. This study was aimed at investigating whether a key mediator of exocytosis soluble N-ethylmaleimide sensitive factor (NSF) attachment protein alpha (alpha SNAP), regulates epithelial junctions. alpha SNAP was enriched at apical junctions in SK-CO15 and T84 colonic epithelial cells and in normal human intestinal mucosa. siRNA-mediated knockdown of alpha SNAP inhibited AJ/TJ assembly and establishment of the paracellular barrier in SK-CO15 cells which was accompanied by a significant down-regulation of p120-catenin and E-cadherin expression. A selective depletion of p120 catenin effectively disrupted AJ and TJ structure and compromised the epithelial barrier. However, overexpression of p120 catenin did not rescue the defects of junctional structure and permeability caused by alpha SNAP knockdown thereby suggesting the involvement of additional mechanisms. Such mechanisms did not depend on NSF functions or induction of cell death, but were associated with disruption of the Golgi complex and down regulation of a Golgi-associated guanidine nucleotide exchange factor, GBF1. These findings suggest novel roles for alpha SNAP in promoting the formation of epithelial AJs and TJs by controlling Golgi-dependent expression and trafficking of junctional proteins.