FIG. 5.

Schematic illustration of the formation of membrane networks such as the network shown in Fig. 3. (Panel 1) Cytoplasmic membrane of wild-type cell or overproducing cell when the levels of Tsr expression are not very high. (Panel 2) Invaginations of the cytoplasmic membrane at different regions to accommodate the increased levels of Tsr by increasing the ratio of membrane surface to cytoplasmic volume. (Panel 3) Formation of two kinds of interactions between proximal invaginated membranes. The interactions between the cytoplasmic domains (yellow) in adjacent membranes lead to a zipper-like structure with wider spacing (about 270 Å), while the interactions between the periplasmic domains lead to the zipper-like structure with narrower spacing (about 150 Å). Note that the diagram is drawn so that it indicates that not all regions of the membrane are part of the zippered structures. (Panel 4) The rounded regions at the ends of the invaginations are occasionally large enough to accommodate an invagination from a different region of the membrane, as shown by the rounded tubule beginning to emerge from the backside. (Panel 5) The secondary invagination can become large enough that its growth is limited by the boundaries of other surrounding membranes. A cross-sectional view through such a region would have the appearance of a free-standing vesicular region. This diagram corresponds to the tomogram shown in Fig. 4b (rotated clockwise about 70°).