Models for the spatial distribution of protein, lipid and water in gap junction structures have been constructed from the results of the analysis of X-ray diffraction data described here and the electron microscope and chemical data presented in the preceding paper (Caspar, D. L. D., D. A. Goodenough, L. Makowski, and W. C. Phillips. 1977. 74:605-628). The continuous intensity distribution on the meridian of the X-ray diffraction pattern was measured, and corrected for the effects of the partially ordered stacking and partial orientation of the junctions in the X-ray specimens. The electron density distribution in the direction perpendicular to the plane of the junction was calculated from the meridional intensity data. Determination of the interference function for the stacking of the junctions improved the accuracy of the electron density profile. The pair-correlation function, which provides information about the packing of junctions in the specimen, was calculated from the interference function. The intensities of the hexagonal lattice reflections on the equator of the X-ray pattern were used in coordination with the electron microscope data to calculate the two-dimensional electron density projection onto the plane of the membrane. Differences in the structure of the connexons as seen in the meridional profiles and equatorial projections were shown to be correlated to changes in lattice constant. The parts of the junction structure which are variable have been distinguished from the invariant parts by comparison of X-ray data from different specimens. The combination of these results with electron microscope and chemical data provides low-resolution threedimensional representations of the structures of gap junctions.The structural variations detailed in the preceding paper (5) establish that we are looking at not one structure of the gap junction, but at a family of structures. By observing closely related states of a molecular assembly, it is often possible to infer something about the way that transitions occur between states. These molecular rearrangements may be significant in the functional activity of the structure. Furthermore, polymorphism provides a constraint on the interpretation of the diffraction patterns. For example, the connexon structure is likely to be similar in arrays with different lattice constants, although the intensities in the X-ray patterns may be quite different.

X-ray crystallographic methods and electron microscope image analysis have been used to correlate the structure and the chemical composition of gap junction plaques isolated intact from mouse liver. The requirement that the interpretations of X-ray, electron microscope, and chemical measurements be consistent reduces the uncertainties inherent in the separate observations and leads to a unified picture of the gap junction structures. Gap junctions are built up of units called connexons that are hexagonally arrayed in the pair of connected cell membranes. X-ray diffraction and electron microscope measurements show that the lattice constant of this array varies from about 80 to 90 A. Analysis of electron micrographs of negatively stained gap junctions shows that there is significant short range disorder in the junction lattice, even though the long range order of the array is remarkably regular. Analysis of the disorder provides information about the nature of the intermolecular forces that hold the array together.

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