Figure 1.

Diagram illustrating gap junction channels and hemichannels. The head to head docking of two hemichannels forms a gap junction channel. Each hemichannel is oligohexamer of the protein subunits, pannexins (Panxs) or connexins (Cxs). (a) In vertebrates, Cxs form hemichannels and gap junction channels, whereas Panxs are believed to form only hemichannels. (b) Invertebrates express innexins and they can form hemichannels and gap junction channels. Hemichannels are permeable to ions including Ca²⁺, and small molecules including signaling ones such as ATP and NAD⁺. Similarly, gap junction channels are permeable to ions and small molecules including IP₃.