Gap junctions are formed by a hexameric group of proteins called connexins for the transport of low molecular weight proteins from cell to cell. Connexins, which are present in all metazoan organisms, serve diverse functions ranging from control of cell growth and differentiation to electric conduction in excitable tissues. Several mammalian cells with malignant phenotypes exhibit decreased connexin expression and gap junction communication. The pannexin gene family encodes a second class of putative gap junction proteins. Pannexins are highly conserved in invertebrates and mammals, indicating the importance of their gap junctional coupling function. Mammalian Pannexin-1 and Pannexin-3 are closely related, while Pannexin-2 is a more distant relation. Pannexin-1 is a transmembrane protein that forms calcium-permeable gap junctions between adjacent cells and in the endoplasmic reticulum. In erythrocytes, Pannexin-1 forms a mechanosensitive ATP-permeable channel in the nonjunctional plasma membrane.
