presented his group's efforts to understand how organelle shapes are formed and maintained. The fi ndings identify two protein families that create tubules out of ER membrane. A sphere is the most stable membrane shape; deformations that increase membrane curvature cost energy and must be actively stabilized. Rapoport and colleagues, including Gia Voeltz, used an in vitro system for ER network formation to identify factors that create curved membrane shapes. With just membranes, salt, and GTP, their system produces a network of ER tubules.< I D > J C B 1 7 2 4 M R _ F i g. 1. e p s < / I D > This simple, self-contained system was not ideal for identifying the shape-creating components. "When we fi rst saw [that] everything was already in the membrane," said Rapoport in his talk, "we thought, 'what a bummer.'" But the group found a way around this diffi culty by using small molecule inhibitors to block in vitro ER formation, and then identifying the inhibitors' targets. One such target was an integral membrane protein called Reticulon4a (Rtn4a), previously named for its localization to ER membranes. All eukaryotes express at least one homologue of Rtn4a, and the proteins are the fi rst known markers specifi cally localized to the tubular ER and absent from sheets. Cells overexpressing Rtn proteins formed more tubules, but loss of the two yeast members did not prevent tubule formation under normal conditions. Only when mutant cells were subjected to osmotic stress were their tubules lost. Rtn proteins form homo-and hetero-oligomers, so the group fi gured that another Rtn-interacting protein might be required for tubule formation. Indeed, they found that Rtn pulled down another ubiquitous integral membrane protein called DP-1. Loss of both the yeast DP-1 and the more abundant of its two Rtns now blocked tubule formation. The group has proposed that Rtn and DP-1 might be wedge-shaped, with their wider sides in the outer membrane leafl et. The presence of these proteins would thus favor a highly curved membrane. They now plan to test whether purifi ed Rtn and DP-1 can turn liposomes into tubules. NL

Elimination of misfolded proteins from the endoplasmic reticulum (ER) by retro-translocation is an important physiological adaptation to ER stress. This process requires recognition of a substrate in the ER lumen and its subsequent movement through the membrane by the cytosolic p97 ATPase. Here we identify a p97-interacting membrane protein complex in the mammalian ER that links these two events. The central component of the complex, Derlin-1, is a homologue of Der1, a yeast protein whose inactivation prevents the elimination of misfolded luminal ER proteins. Derlin-1 associates with different substrates as they move through the membrane, and inactivation of Derlin-1 in C. elegans causes ER stress. Derlin-1 interacts with US11, a virally encoded ER protein that specifically targets MHC class I heavy chains for export from the ER, as well as with VIMP, a novel membrane protein that recruits the p97 ATPase and its cofactor.

The endoplasmic reticulum (ER) consists of the nuclear envelope and a peripheral network of tubules and membrane sheets. The tubules are shaped by the curvature-stabilizing proteins reticulons and DP1/Yop1p, but how the sheets are formed is unclear. Here we identify several sheet-enriched membrane proteins in the mammalian ER, including proteins that translocate and modify newly synthesized polypeptides, as well as coiled-coil membrane proteins that are highly upregulated in cells with proliferated ER sheets, all of which are localized by membrane-bound polysomes. These results indicate that sheets and tubules correspond to rough and smooth ER, respectively. One of the coiled-coil proteins, Climp63, serves as a “luminal ER spacer” and forms sheets when overexpressed. More universally, however, sheet-formation appears to involve the reticulons and DP1/Yop1p, which localize to sheet edges and whose abundance determines the ratio of sheets to tubules. These proteins may generate sheets by stabilizing the high curvature of edges.