Connexon

linked to Direct contact communication of cells

• Connexins are the bricks: A connexin is a single protein. By itself, it is just one building block. • Connexon is the half-door: When six connexins come together, they form a hollow tube with a hole in the middle. This tube is called a connexon (or a hemichannel). It acts like a door built into the wall of one cell. • The Gap Junction is the tunnel: To actually connect two cells, the half-door (connexon) of one cell lines up perfectly with a half-door (connexon) of the neighboring cell. They lock together to create an open tunnel.

Six connexin transmembrane protein subunits assemble to form a hemichannel called a connexon.

Key Structural Facts

• Connexins: Integral membrane proteins that span the lipid bilayer four times. • Connexon (Hemichannel): An annular, hexameric assembly of six connexin proteins featuring a central aqueous pore. • Gap Junction Channel: Formed when two opposing connexons from adjacent cells dock end-to-end (totaling 12 connexin subunits).

Classification & Composition

• Homomeric vs. Heteromeric: A connexon is homomeric if all six connexin subunits are identical, or heteromeric if it contains different connexin isoforms. • Homotypic vs. Heterotypic: A complete gap junction is homotypic if both docked connexons are identical, or heterotypic if they differ.

Function

• Direct Transfer: Allows bidirectional flow of ions, secondary messengers, and small metabolites (<1,000–1,200 Daltons) between cytoplasms. • Cellular Coordination: Synchronizes electrical activity and metabolic responses in tissues like the heart, brain, and smooth muscle.