Type IV secretion (T4S) systems form the most common and versatile class of secretion systems in bacteria, capable of injecting both proteins and DNAs into host cells. T4S systems are typically composed of 12 components that form two major assemblies: the inner membrane complex embedded in the inner membrane and the core complex embedded in both the inner and outer membranes. Here we present the 3.3 Å resolution cryo-electron microscopy model of the T4S system core complex from Xanthomonas citri , a phytopathogen that utilizes this system to kill bacterial competitors. An extensive mutational investigation was performed to probe the vast network of protein-protein interactions in this 1.13 MDa assembly. This structure expands our knowledge of the molecular details of T4S system organization, assembly and evolution.

Background: GAC supplies for increased metabolic needs of tumors because of exclusive localization and kinetic properties. Results: Higher than tetramer oligomers are the active form in in vitro and in cellular assays. Bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl)ethyl sulfide disrupts oligomers. Conclusion: A novel molecular mechanism for GAC activation is proposed. Significance: The data affect the development of therapies targeting GAC in tumors, with emphasis on allosteric inhibitors.

Septins are GTP binding proteins considered to be novel components of the cytoskeleton. They polymerize into filaments based on hexameric or octameric core particles in which two copies of either three or four different septins, respectively, assemble into a specific sequence. Viable combinations of the 13 human septins are believed to obey substitution rules in which the different septins involved must come from distinct subgroups. The hexameric assembly, for example, has been reported to be SEPT7–SEPT6–SEPT2–SEPT2–SEPT6–SEPT7. Here, we have replaced SEPT2 by SEPT5 according to the substitution rules and used transmission electron microscopy to demonstrate that the resulting recombinant complex assembles into hexameric particles which are inverted with respect that predicted previously. MBP‐SEPT5 constructs and immunostaining show that SEPT5 occupies the terminal positions of the hexamer. We further show that this is also true for the assembly including SEPT2, in direct contradiction with that reported previously. Consequently, both complexes expose an NC interface, as reported for yeast, which we show to be more susceptible to high salt concentrations. The correct assembly for the canonical combination of septins 2‐6‐7 is therefore established to be SEPT2–SEPT6–SEPT7–SEPT7–SEPT6–SEPT2, implying the need for revision of the mechanisms involved in filament assembly.