Molecular organization of gap junction membrane channels. 
Gap junctions regulate a variety of cell functions by creating a conduit between two apposing tissue cells.  Gap junctions are unique among membrane channels.  Not only do the constituent membrane channels span two cell membranes, but the intercellular channels pack into discrete cell-cell contact areas forming in vivo closely packed arrays.  Gap junction membrane channels can be isolated either as two-dimensional crystals, individual intercellular channels, or individual hemichannels.  The family of gap junction proteins, the connexins, create a family of gap junctions channels and structures.  Each channel has distinct physiological properties but a similar overall structure.  This review focuses on three aspects of gap junction structure: (1) the molecular structure of the gap junction membrane channel and hemichannel, (2) the packing of the intercellular channels into arrays, and (3) the ways that different connexins can combine into gap junction channel structures with distinct physiological properties.  The physiological implications of the different structural forms are discussed.