Structure of the gap junction channel and its implications for its biological functions

Abstract

Gap junctions consist of arrays of intercellular channels composed of integral membrane proteins called connexin in vertebrates. Gap junction channels regulate the passage of ions and biological molecules between adjacent cells and, therefore, are critically important in many biological activities, including development, differentiation, neural activity, and immune response. Mutations in connexin genes are associated with several human diseases, such as neurodegenerative disease, skin disease, deafness, and developmental abnormalities. The activity of gap junction channels is regulated by the membrane voltage, intracellular microenvironment, interaction with other proteins, and phosphorylation. Each connexin channel has its own property for conductance and molecular permeability. A number of studies have tried to reveal the molecular architecture of the channel pore that should confer the connexin-specific permeability/selectivity properties and molecular basis for the gating and regulation. In this review, we give an overview of structural studies and describe the structural and functional relationship of gap junction channels.

Fig. 1

Overall structure of the human Cx26 gap junction channel in ribbon representation. a Side view of the Cx26 gap junction channel with the locations of plasma membranes and scale of each region. Each subunit is colored differently, and those associated with the crystallographic two-fold axis are in the same color. b Top view of the Cx26 gap junction channel representing the arrangement of the transmembrane helices and the N-terminal helix. The channel has a hexagonal appearance with the largest outer diameter of ~90 Å and a pore entrance of ~40 Å

Fig. 2

Wall-eye stereo view of the Cx26 monomer in ribbon representation. Each region is colored differently, and the upper arrows indicate the pore side and the lipid side. Three disulfide bonds in the extracellular region are shown in stick representation. Unobserved regions in the cytoplasmic loop and the C-terminal tail are represented by dashed lines

Fig. 3

Ribbon representation of the Cx26 monomer is colored according to conservation of residues in the connexin family [104] (the gradient from white to violet indicates increasingly variable residues)

Fig. 4

Structural organization of Cx26 monomer and hexamer. a Topological map of mutations associated with deafness and skin disease, adapted from [51]. b Intramolecular interactions that stabilize the monomer structure of Cx26. c Intermolecular interactions between two neighboring monomers in a connexon. Each interaction is shown in the enlarged insets

Fig. 5

Interactions between apposing connexons. Interactions of E1 and E2 are each shown in the enlarged insets

Fig. 6

Pore architecture of the Cx26 gap junction channel. Left Cx26 gap junction channel is rendered as surface drawing and sectioned along the six-fold axis of symmetry, showing the surface potential distribution of the channel interior. Right The pore diameter is illustrated along the six-fold axis generated using the HOLE program [179]

Fig. 7

Sequence alignment of human connexins in the pore-lining region. Amino acid residues of Cx26 from the N-terminus to amino acid 65 are aligned with CLUSTALW [180]. Secondary structures and numbering of residues of Cx26 are represented at the top. Asterisks indicate pore-exposing residues in Cx26. Figures are created with ESPript [181] and manually modified

Fig. 8

The structure of pore funnel and pore plug. a The six NTHs form pore funnel, which is stabilized by circular hydrogen bond network (red dashed lines) at the bottom of it and attached to the inner wall of the channel by hydrophobic interactions (orange dashed lines). These interactions are formed between neighboring monomers. b Superposition of the atomic model of wild-type Cx26 gap junction channel (ribbon representation: green) into the electron density map of Met34Ala mutant Cx26 (surface representation: gray)

Fig. 9

Plug gating model for transjunctional voltage-dependent gating of the Cx26 gap junction channel. When there is no difference in membrane voltages between two neighboring cells (a), NTHs form the pore funnel and attach to TM1 by hydrophobic interactions. When there is a difference in membrane voltages between two cells (b), the positive electric field pulls up Asp2, which is exposed to the pore, in the cytoplasmic direction, releasing NTHs from TM1. Once released, NTHs will assemble on the top of the pore and form a so-called “plug” structure