The inside and outside of gap-junction membranes visualized by deep etching.

We have viewed the membrane specializations that occur at gap junctions from the inside and outside of cells in replicas of quick-frozen and deep-etched samples. Gap junctions were split to expose the normally apposed outside surfaces of their membranes, which displayed uniform 8-9 nm protrusions with central pores. Such pores were also observed in the protoplasmic-face particles of freeze-fractured gap junctions, even after the junctions were induced to crystallize by treatment with metabolic inhibitors or by homogenization. Crystallized junctions have been shown to be in the closed, high-resistance state; hence the channel-closing mechanism must not be located in the regions viewed so far. In washed-out broken cells, the inner surfaces of gap junctions possess smooth surfaces with no visible pores. These surfaces are devoid of special undercoatings of cytoskeletal elements, suggesting that crystallization observed during uncoupling is an intramembrane phenomenon. Hypertonicity, in itself, may produce the same sort of hexagonal crystallization of gap-junction components that is usually observed after uncoupling.