A Crystallogral structure is described for the Mg2+-BeF3--bound receiver domain of Sinorhizobium meliloti DctD bearing amino acid substitution E121K. Differences between the apo- and ligand-bound active sites are similar to those reported for other receiver domains. However, the off and on states of the DctD receiver domain are characterized by dramatically different dimeric structures, which supports the following hypothesis of signal transduction. In the off state, the receiver domain and coiled-coil linker form a dimer that inhibits oligomerization of the AAA+ ATPase domain. In this conformation, the receiver domain cannot be phosphorylated or bind Mg2+ and BeF3-. Instead, these modifications stabilize an alternative dimeric conformation that repositions the subunits by approximately 20 A, thus replacing the a4-b5-a5 interface with an a4-b5 interface. Reoriented receiver domains permit the ATPase domain to oligomerize and stimulate open complex formation by the s54 form of RNA polymerase. NtrC, which shares 38% sequence identity with DctD, works differently. Its activated receiver domain must facilitate oligomerization of its ATPase domain. Significant differences exist in the signaling surfaces of the DctD and NtrC receiver domains that may help explain how triggering the common two-component switch can variously regulate assembly of a AAA+ ATPase domain.

We report the crystal structure of a fragment of Sinorhizobium meliloti DctD, a bacterial enhancer binding protein, at 1.7 Å. The fragment contains the protein's two‐component receiver module and adjacent linker, which in the native protein joins the receiver domain to a σ54‐dependent ATPase domain. The structure reveals a novel dimerization surface, which sequence analysis indicates is common to 4.5% of the known two‐component receiver domains. Genetic, biochemical, and structural data for amino acid substitution variants indicate that the dimer is necessary to inhibit the basal activity of the ATPase domain. The dimerization element is thus needed to maintain the “off” state, and changes within it may signal activation. Analytical ultracentrifugation data for the phosphorylated fragment of DctD appear to rule out the simple model that signaling is mediated via monomerization of the receiver domain.

Rolling neutrophils form tethers with submicron diameters. Here, we report that these tethers detach, forming elongated neutrophil-derived structures (ENDS) in the vessel lumen. We studied ENDS formation in mice and humans in vitro and in vivo. ENDS do not contain mitochondria, endoplasmic reticulum, or DNA, but are enriched for S100A8, S100A9, and 57 other proteins. Within hours of formation, ENDS round up, and some of them begin to present phosphatidylserine on their surface (detected by annexin-5 binding) and release S100A8–S100A9 complex, a damage-associated molecular pattern protein that is a known biomarker of neutrophilic inflammation. ENDS appear in blood plasma of mice upon induction of septic shock. Compared with healthy donors, ENDS are 10–100-fold elevated in blood plasma of septic patients. Unlike neutrophil-derived extracellular vesicles, most ENDS are negative for the tetraspanins CD9, CD63, and CD81. We conclude that ENDS are a new class of bloodborne submicron particles with a formation mechanism linked to neutrophil rolling on the vessel wall.