Structure of Cardiac Gap Junction Intercellular Channels

Abstract

Gap junction proteins, termed connexins, constitute a multigene family of polytopic membrane channel proteins that have four hydrophobic transmembrane domains with the N- and C-termini located on the cytoplasmic membrane face. The principal gap junction protein in the heart, α1connexin (also designated Cx43), mediates action potential propagation between cells in order to synchronize cardiac contraction. α1connexin channels are concentrated in gap junction plaques located in the intercalated disks. The intercellular channel is formed by the docking of two hemi-channels, termed connexons, formed by a ring of six 43-kDa α1connexin subunits. Each subunit is asymmetric with an axial ratio of 4–5:1 with ∼20 Å extending into the extracellular gap ∼50 Å spanning the lipid bilayer and ∼50 Å extending into the cytoplasmic space. We have recently grown two-dimensional crystals of a recombinant C-terminal truncation mutant of α1connexin (designated α1Cx263T) that are ordered to better than 7 Å resolution. Projection density maps derived by electron cryocrystallography revealed that the intercellular channel is lined by six α-helices, and there is a second ring of six α-helices at the interface with the membrane lipids. These rings of α-helices are staggered by 30°, which predicts that the two connexons in the channel are staggered by 30° such that each connexin subunit in one connexon interacts with two subunits in the apposed connexon. Such a quaternary arrangement may confer stability in the docking of the connexons to form a tight electrical seal for intercellular current flow during cardiac conduction.