FIG. 1.

Gap-junction channels in the plasma membrane. The family of proteins known as connexins constitutes the structural and functional unit of gap-junctional intercellular communication. Oligomerization of six connexin proteins forms a connexon or hemichannel (49, 162). In response to stimuli such as phosphorylation or Ca²⁺, the associated six connexin subunits may coordinately change configuration to open the hemichannel and allow movement of paracrine signaling molecules such as ATP (70, 163). Hemichannels may diffuse laterally to the junctional membrane where they are in a position to dock with another hemichannel on the apposed membrane of the adjacent cell to complete a gap-junction channel (49, 162).