tER sites are enriched in COPII vesicles containing proteins destined for the Golgi. In fly S2 cells, tER sites are found adjacent to well-defined Golgi stacks. The authors have found that tER organization in S2 cells depends on dp115, of which mammalian homologues are necessary for vesicle transport and Golgi structure. In the absence of dp115, small tER sites that were no longer spatially associated with the Golgi were scattered throughout the cytoplasm.
As expected, Golgi stacks were fragmented into vesicles and tubules in dp115-depleted cells. Transport was not inhibited, however, so at least in S2 cells, Golgi structure and function are not necessarily linked. Golgi fragmentation did not cause the tER disorganization, as depletion of syntaxin 5 disrupted Golgi stacks but not tERs. Rather, dp115 probably has a separate function as a component of a recently proposed matrix surrounding tERs. The authors found that dp115 is as abundant in the tER as it is in the Golgi complex. The protein may help build the tER through its interaction with the COPII coat.
According to the cisternal maturation model of Golgi structure, tER sites fuse to form Golgi cisternae that continually mature and ultimately bud off at late Golgi elements. A matrix around the tER could explain how enough vesicles are tethered together to reach a threshold that promotes formation of cisternae. ▪