Gap Junctions

Connexins are transmembrane proteins that assemble into hexameric structures called connexons. Six connexin molecules form a hemichannel, which can be homomeric (identical connexins) or heteromeric (different connexins). When connexons from adjacent cells pair up, they create complete channels that form gap junction plaques, enabling molecular exchange between cells.

Gap junctions control molecular transport through channel selectivity determined by connexin diversity and combinations. Transport is further regulated by opening and closing channels in response to stimuli including voltage differences, calcium ion concentration, and pH. This dynamic regulation allows cells to selectively exchange ions, secondary messengers, sugars, and other small molecules based on cellular needs.

Invertebrate gap junctions use innexins (invertebrate connexins) instead of connexins. While innexins share a similar transmembrane structure, they lack sequence homology with vertebrate connexins. Crucially, innexons require eight units to form a hemichannel, whereas vertebrate connexons require only six units, reflecting structural differences between invertebrate and vertebrate communication channels.

Connexons are synthesized on rough endoplasmic reticulum, transported through the Golgi network, and delivered to the plasma membrane via secretory vesicles. Once at the membrane, connexons freely diffuse until encountering other connexons, forming clusters. New connexons continuously add to cluster peripheries while old connexons at the center are endocytosed and degraded, creating dynamic, constantly remodeled gap junction plaques.

Gap junctions in heart muscle respond to voltage differences across cell membranes. This voltage sensitivity allows rapid ion flow synchronization between adjacent muscle cells, coordinating their electrical activity. This synchronized ion exchange generates rhythmic contractions throughout the heart, enabling coordinated pumping action essential for effective circulation.

Animal cells use gap junctions where plasma membranes directly contact adjacent cells. Plant cells, separated by cell walls preventing direct membrane contact, instead use plasmodesmata—tubular channels 20-40 nanometers in diameter where one cell's membrane is continuous with the adjacent cell's membrane, allowing contiguous cytoplasm and regulated molecular exchange.

Connexin diversity enables channel selectivity for specific molecules through different protein combinations. Homomeric connexons use identical connexins, while heteromeric connexons combine different connexins, creating varied channel properties. This molecular diversity allows different cell types and tissues to establish specialized communication channels suited to their specific physiological requirements and signaling needs.