Bridges linking gap junction particles extracellularly: a freeze-etching rotary-shadowing study of split junctions.

The nature of the interaction between neighboring gap junction particles and the mechanism involved in particle crystallization are still unclear. We describe here interparticle bridge-like structures which could participate in the mechanism of gap junction particle aggregation and pattern determination. Gap junction membranes of rat liver, pulled apart by vascular perfusion with hypertonic sucrose, were freeze-fractured in deionized water, etched at - 100 degrees C for 8 min and rotary-shadowed at a 32 degrees angle. At the extracellular true surface of the junctions (ES-surface), the particles appear as 7.0 to 7.5 nm rings often resolvable into six radially arranged subunits. The particles appear linked to each other by filamentous bridges 1.5 to 2.2 nm thick and approximately 1.5 nm long. Some bridges (single bridges) directly interlink neighboring particles while other bridges (multiple bridges) are joined to a particle at one end and to the other bridges at the other end. Bridges are referred to as double or triple bridges if they result from the interaction of two or three bridges respectively. In particles which can be resolved into six subunits, the bridges appear to bind to the subunit tips. Stereo images indicate that the bridges lay in planes lower than the particle summits. The bridges could be either polypeptide chains of the main gap junction protein or peripheral proteins, but the unlikely possibility that they are a shadowing artifact cannot be entirely ruled out yet.