Figure 8.

Model of a proposed mutual feedback relationship between the tER and the Golgi. (Left) In normal cells, tER sites (red) and Golgi structures (green) exchange material by microtubule-directed transport (bidirectional arrows). ERGIC elements arise at tER sites and move inward to the juxtanuclear Golgi, while retrograde traffic from the Golgi moves outward toward the cell periphery. Retrograde transport events terminate either at tER sites (as drawn) or at random locations on the ER. This recycling of proteins to the ER causes tER sites to proliferate. Because most of the retrograde transport events terminate near their point of origin, ER membranes in the Golgi region contain a high concentration of recycling proteins and a correspondingly high density of tER sites. (Right) When microtubules are disrupted with nocodazole, each Golgi structure receives input only from adjacent tER sites, and the number of these sites determines the size and stability of the Golgi structure. Conversely, membrane recycling from a given Golgi structure induces a localized proliferation of the tER. This positive feedback loop generates intermediate-sized Golgi structures that are next to tER clusters.