Connexon

A connexon, also known as a hemichannel, is an assembly of six protein subunits called connexins, forming a pore-like structure in the plasma membrane of a cell. Connexons are essential components of gap junctions, specialized intercellular channels that directly connect the cytoplasm of two adjacent cells.

When two connexons, each contributed by a neighboring cell, align and dock end-to-end in the intercellular space, they create a complete gap junction channel. This channel allows the direct passage of ions, small molecules (up to ~1 kDa), and electrical signals between the connected cells.

The specific type of connexins that compose a connexon determines the channel's properties, including its permeability, selectivity for different molecules, and sensitivity to regulatory factors such as pH and voltage. Different cell types express different combinations of connexins, resulting in diverse gap junction channel properties tailored to specific physiological functions.

Gap junctions and their constituent connexons play critical roles in various biological processes, including:

• Electrical and metabolic coupling: Facilitating the coordinated activity of excitable cells, such as neurons and cardiomyocytes, and enabling the sharing of nutrients and signaling molecules between cells in tissues.
• Cellular communication: Allowing cells to communicate directly with each other, bypassing the need for extracellular signaling molecules.
• Tissue homeostasis: Contributing to the maintenance of tissue integrity and function by coordinating cell growth, differentiation, and response to stress.
• Embryonic development: Playing a crucial role in coordinating cell fate and morphogenesis during development.

Dysfunction of connexons and gap junctions has been implicated in a variety of diseases, including cardiac arrhythmias, neurodegenerative disorders, and certain types of cancer.