Gap junction structures. IV. Asymmetric features revealed by low-irradiation microscopy

摘要

Micrographs of mouse liver gap junctions, isolated with detergents, and negatively stained with uranyl acetate, have been recorded by low- irradiation methods. Our Fourier-averaged micrographs of the hexagonal junction lattice show skewed, hexameric connexons with less stain at the threefold axis than at the six indentations between the lobes of the connexon image. These substructural features, not clearly observed previously, are acutely sensitive to irradiation. After an electron dose less than that normally used in microscopy, the image is converted to the familiar doughnut shape, with a darkly stained center and a smooth hexagonal outline, oriented with mirror symmetry in the lattice. Differences in appearance among 25 reconstructed images from our low- irradiation micrographs illustrate variation in staining of the connexon channel and the space between connexons. Consistently observed stain concentration at six symmetrically related sites approximately 34 A from the connexon center, 8 degrees to the right or left of the (1, 1) lattice vector may reveal an intrinsic asymmetric feature of the junction structure. The unexpected skewing of the six-lobed connexon image suggeststhat the pair of hexagonal membrane arrays that form the junction may notbe structurally identical. Because the projected image of the connexon pairitself appears mirror symmetric, each pair may consist of two identicalconnexon hexamers related by local (noncrystallographic) twofold axes inthe junctional plane at the middle of the gap. All connexons may bechemically identical, but their packing in the hexagonal arrays on the twosides of the junction appears to be nonequivalent.