Differences between cytoplasmic surfaces of deep-etched heart and liver gap junctions.

We have compared the ultrastructures of the cytoplasmic surfaces (CS) of isolated, glutaraldehyde-fixed gap junctional pellets from rat ventricles and liver by rapid freezing on a liquid helium-cooled surface, freeze fracture, deep etching, and double-axis rotary replication (J. Microsc. Oxford 137: 121-123, 1984). Deep-etched unproteolyzed cardiac junctions [protein subunit relative molecular wt (Mr) 44,000-47,000], isolated with phenylmethylsulfonylfluoride (PMSF) [Am. J. Physiol. 246 (Heart Circ. Physiol. 15): H865-H875, 1984; C.K. Manjunath, G.E. Goings, and E. Page. Proteolysis of cardiac gap junctions during their isolation from rat hearts. J. Membr. Biol. In press.] had particulate CS, while proteolyzed cardiac junctions (subunit Mr 29,500) made without PMSF and liver junctions (Mr 28,000) made with or without PMSF had nonparticulate CS. Taken together with our previous findings that electron micrographs of thin-sectioned isolated unproteolyzed cardiac junctions have urea-resistant fuzzy CS coatings [Am. J. Physiol. 246 (Heart Circ. Physiol. 15): H865-H875, 1984], that proteolyzed cardiac junctions and isolated liver junctions lack this fuzzy layer, and that the CS of in situ cardiac junctions is fuzzy [J. Membr. Biol. 78: 147-155, 1984; Am. J. Physiol. 246 (Heart Circ. Physiol 15): H865-H875, 1984], and published data that in situ freeze-etched liver gap junctions are nonparticulate (Cell 30: 395-406, 1982), these new observations strongly indicate that CS components of heart and liver gap junction proteins are structurally different.