FIG. 4.

(a) Segmented representation of a small region from a cellular tomogram. (b) Slice of the tomogram. The region indicated by the box (width, 230 nm) corresponds to a slice in the region that was segmented. Aside from the interaction between the cytoplasmic domains (yellow) in apposing bilayers, a complementary interaction between the periplasmic domains (blue) also appears to be likely. The uniformity in spacing between the rounded vesicular region and the invaginated membrane surrounded by it suggests that the entire membrane network is stabilized by interactions at both ends of the receptor. The segmented region is composed of two sets of stacked membranes. The lower membrane sheet is wrapped around a tubular invagination (see panel 4 in Fig. 5 for a schematic representation of the geometry). The segmentation was carried out to be faithful to the density in the tomogram and is therefore noisy, as seen, for example, in the patchy appearance of the rounded end of the tubular invagination.