A freeze-fracture paradigm of the mechanism for delivery and insertion of gap junction particles into the plasma membrane.

Formation of intercellular gap junctions between cultured bovine aorta endothelial cells was studied by examination of freeze-fracture replicas of endothelial plasma membranes. Newly divided cells in the endothelial monolayer displayed numerous plaque-like aggregates of intramembrane particles situated on the PF fracture face of the plasmalemma interpreted to represent gap junctions in various stages of assembly. Closely associated with these particle aggregates are numerous small circular PF face depressions (or EF face volcano-like protrusions) that are presumed to result from fusion of cytoplasmic vesicles with the plasma membrane. In most cases, intramembrane particles, identical to those that comprise the developing gap junctions, appear intimately associated with the vesicle fusion sites and with the forming junction particle aggregates; a morphological relationship that strongly suggests the gap junction particles have emanated from the fusion sites. These observations are suggested to constitute morphological evidence for insertion of gap junction intramembrane particles into the plasma membrane by fusion of particle-bearing cytoplasmic vesicle couriers with the plasmalemma.