Three-dimensional structure of a recombinant gap junction membrane channel

Gap junction membrane channels mediate electrical and metabolic coupling be tween adjacent cells. The structure of a recombinant cardiac gap junction c hannel was determined by electron crystallography at resolutions of 7.5 ang stroms in the membrane plane and 21 angstroms in the vertical direction. Th e dodecameric channel was formed by the end-to-end docking of two hexamers, each of which displayed 24 rods of density in the membrane interior, which is consistent with an alpha-helical conformation for the four transmembran e domains of each connexin subunit. The transmembrane alpha-helical rods co ntrasted with the double-layered appearance of the extracellular domains. A lthough not indicative for a particular type of secondary structure, the pr otein density that formed the extracellular vestibule provided a tight seal to exclude the exchange of substances with the extracellular milieu.