Golden Gate Cells: The American Society for Cell Biology San Francisco, CA December 10–14, 2005

Cytoplasmic dynein is a AAA ATPase with a ring structure at each head.) 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. 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 T ransport within a cell occurs with the help of three classes of motor proteins: myosin, kinesin, and dy-nein, which carry their cargo along cyto-skeletal tracks. Conventional kinesin and myosin V move processively—that is, they remain bound to their tracks for many steps. But the minus end–directed motor dynein, because of its large …