Gap junction structures. VIII. Membrane cross-sections

Profiles of negatively stained gap junctions have been measured by grid sectioning. After normal levels of electron irradiation, the membrane thickness shrinks to about half that of unirradiated controls, but no shrinkage occurs in the hexagonal lattice plane. Even under low irradiation conditions, there is significant thinning of the membranes. Edge views, in which rows of connexons are aligned parallel to the beam, were obtained from grid sections, folds in normal negatively stained specimens, and sections of a positively stained specimen. Averaging these micrographs with the translational and mirror symmetry of the projected lattice image displays conserved and variable features in the stain distribution of different specimens. Variations in the relative amount of negative stain in the gap at the surfaces and in the channel are uncorrelated with the irradiation but appear to depend on the local staining conditions and the integrity of the connexons. The dimensions measured from previously unirradiated grid sections, folds, and positively stained sections are in accord with x-ray diffraction measurements. Radiation-induced shrinkage can be accounted for by mass loss principally from the membrane bilayer. Disordering of the surface structure appears to be correlated with the radiation sensitivity of the bilayer; in contrast, the gap structure is well preserved under a variety of conditions.