Overcoming barriers in the study of tight junction functions: from occludin to claudin

Tight junctions (TJs) are essential structures for the physiological functions of epithelial and endothelial cells, and have been suggested to have both barrier and fence functions. Tight junctions create a primary barrier to the diffusion of solutes through the paracellular pathway, and also function as a fence between apical and basolateral membrane domains, to create and maintain cell polarity of epithelial and endothelial cells. Several peripheral membrane proteins have been shown to be concentrated at the cytoplasmic surface of TJs. However, TJ-specific integral membrane proteins had not been identified until recently, and the lack of information concerning TJ-specific integral membrane proteins has hampered a more direct assessment of the function of TJs at the molecular level. Here, we present an overview of current progress in the identification of TJ-specific integral membrane proteins.