Connexins: A Brief Overview

Connexins, elementary protein units of gap junctions, make intercellular and membrane channels that work as conduits for ions and larger molecules >1 kDa. Electrically, cells well coupled by gap junctions display a relatively uniform resting potential. In excitable tissues, gap junctions are the pathway for electrotonic propagation of action potentials. When hemichannels open in the membrane, connecting the cytoplasm and the extracellular space, the resting potential could collapse, and action potential propagation be impaired. Because connexin channels are permeated by large molecules, gap junctions are assumed to synchronize cellular functions by the sharing of second messengers, metabolites, and other substances. In turn, hemichannel opening could allow the escape of those same substances, and the uptake of extracellular molecules. Separately from their channel function, parts of the connexin molecule can induce cellular changes that suggest protein-protein interactions with the cytoskeleton, regulatory pathways, and the genomic machinery of the cells. This minireview gives an overview of connexins and discusses some of the outstanding issues in the field.