Gap junctions are collections of oligomeric membrane proteins (connexons), which interact across the space between neighbouring cells to form continuous cell-to-cell pathways for ions and small molecules. The connexon is constructed from six identical subunits, arranged symmetrically in the plane of the membrane and delineating the channel along their common sixfold axis. The subunits are rod-shaped and 7-8 nm long; they protrude about 1.5 nm into the extracellular space, but somewhat less into the cell interior. Their cross-section within the membrane corresponds most closely to that of four closely packed alpha-helical rods. The channel is narrowest near the cytoplasmic surface and widest in the extracellular region. Changes between alternative quaternary configurations are most pronounced in the cytoplasmic region, and involve a coordinated tilting of the subunits, predominantly tangential to the central symmetry axis. The observed molecular details suggest that switching between open and closed states of the channel may entail a cooperative mechanism in which a localized effect induced by ligand binding triggers a long-range concerted rearrangement of the subunits. Other membrane channels have similar molecular designs and may act in an analogous way.