Sequestration puts Drp1 on furlough

A potato virus makes its move T ilsner et al. describe how three viral proteins combine to help newly synthesized viruses spread to neighboring plant cells. Plant viruses move between cells through narrow channels in the cell wall called plasmodesmata. Potato virus X, which replicates on the surface of the endoplasmic reticulum (ER), encodes three proteins—TGB1, 2, and 3—that promote the virus' transport into neighboring cells, but the precise function of each of these " movement proteins " is unclear. Tilsner et al. found that TGB2 and TGB3 established clusters of ER membranes that capped the entrances to plasmodesmata. These caps appeared to be sites of viral replication because they contained the virus' RNA polymerase and RNA that was not yet encased by viral coat proteins. In addition, TGB2 and TGB3 recruited TGB1 to plasmodesmata, where it helped insert viral coat proteins into the intercellular channel. The authors think that newly synthesized viral RNAs are encased by coat proteins and quickly traffi cked into neighboring cells. Thus, by compartmentalizing viral replication to plasmo-desmata, the movement proteins allow the potato virus to rapidly spread before the plant's defense mechanisms can shut the pathogen down. Lead author Jens Tilsner now wants to investigate how TGB1 interacts with the viral coat protein in order to insert it into the plasmodesmata channel. A mature view of the Golgi R izzo et al. demonstrate that Golgi-resident enzymes recycle backwards to promote the maturation of cargo-containing Golgi cisternae. In mammalian cells, the Golgi apparatus consists of stacks of fl at, membrane-bound compartments called cisternae. How secretory proteins move through the stack—from the cis side near the ER to the trans side where they are sorted to their fi nal des-tination—is unclear. In one model, secretory cargo is transported in vesicles from one cisterna to the next, with each Golgi compartment stably maintaining its own composition. An alternative view is that secretory proteins remain in place while the composition of the cisterna changes around them; Golgi resident enzymes are recycled back to earlier cisternae so that each compartment gradually matures and acquires the composition of the trans-Golgi. To distinguish between these possibilities, Rizzo et al. generated a version of the cis-/medial Golgi enzyme mannosidase I (MANI) that could reversibly polymerize into a large network incapable of being incorporated into recycling vesicles or tubules. Just like the endogenous enzyme, monomeric MANI localized in cis and medial cisternae and …