Assembly of heteromeric connexons in guinea-pig liver en route to the Golgi apparatus, plasma membrane and gap junctions

Guinea-pig liver gap junctions are constructed from approximately equal amounts of connexins 26 and 32. The assembly of these connexins into connexon hemichannels and gap junctions was studied using antibodies specific to each connexin. Intracellular membranes were shown to contain low amounts of connexin 26 relative to connexin 32 in contrast to the equal connexin ratios detected in lateral plasma membranes and gap junctions. Assembly of gap junctions requires oligomerization of connexins into connexons that may be homomeric or heteromeric. Immunoprecipitation using antibodies to connexins 26 and 32 showed that liver gap junctions were heteromeric. A chemical cross-linking procedure showed that connexons solubilized from guinea-pig liver gap junctions were constructed of hexameric assemblies of connexin subunits. The intracellular site of oligomerization of connexins was investigated by velocity sedimentation in sucrose-detergent gradients. Oligomers of connexins 26 and 32 were extensively present in Golgi membranes and oligomeric intermediates, especially of connexin 26, were detected in the endoplasmic reticulum-Golgi intermediate subcellular fraction. Two intracellular trafficking pathways that may account for the delivery of connexin 26 to the plasma membrane and explain the heteromeric nature of liver gap junctions are discussed.