The fellowship of the Z ring

S UMOylation of the small GTPase ARL-13 helps target signaling receptors into primary cilia, Li et al. reveal. Primary cilia are microtubule-based membrane protru-sions that serve as hubs for multiple signaling pathways. In C. elegans, ARL-13 promotes cilia assembly by coordinating intrafl agellar transport along ciliary microtubules, and the GTPase is also required for the correct localization of signaling receptors to the ciliary membrane. In a yeast two-hybrid screen for ARL-13 binding partners, Li et al. identifi ed UBC-9, an enzyme that conjugates the small, ubiq-uitin-like modifi er SUMO onto target proteins. UBC-9 colocalized with ARL-13 in primary cilia and SUMOylated the GTPase near its C terminus. A non-SUMOylatable ARL-13 mutant localized to cilia and restored ciliogenesis in worms lacking wild-type ARL-13. But the rescued cilia lacked membrane proteins like the mechano-sensor polycystin-2, indicating that ARL-13 SUMOylation is required for the proper ciliary localization of signaling receptors. A constitutively SUMOylated version of ARL-13 successfully restored both ciliogenesis and receptor targeting. The human homologue of ARL-13, ARL13B, is mutated in the ciliopathy Joubert syndrome. ARL13B was also SUMOylated by UBC9, and this modifi cation was required for the ciliary local-ization of human polycystin-2. Because polycystin-2 is mutated in polycystic kidney disease, Li et al.'s fi ndings may explain why Joubert syndrome patients have cystic kidneys. Mislocalization of other signaling proteins may underlie additional symptoms associated with the disease. Senior author Jinghua Hu now wants to investigate how SUMOylation regulates ARL-13's function in cili-ary targeting. One possibility is that SUMOylation allows ARL-13 to bind an adaptor that links the GTPase to different receptors. B ui et al. investigate how a dy-namin-related GTPase is recruited to the mitochondrial outer membrane to drive mito-chondrial fi ssion. Dynamin binds directly to the plasma membrane to remodel the lipid bilayer during endocytosis. Dynamin-related proteins, in contrast, remodel other cellular membranes after binding to them via specifi c adaptor proteins. The mitochondrial fi ssion protein Dnm1, for example, is recruited to yeast mitochondria by Mdv1, though how Dnm1 interacts with this adaptor protein is unclear. Bui et al. identifi ed several point mutations that prevented Dnm1 from localizing to mitochondria and supporting mitochon-drial fi ssion. The mutations were located in a conserved motif in the " Insert B " domain of Dnm1, a domain found in all dynamin-related proteins but not in dynamin itself, which instead contains a lipid-binding PH domain. Mutations in Dnm1's Insert B domain …