Peripheral blood neutrophils form highly decondensed chromatin structures, termed neutrophil extracellular traps (NETs), that have been implicated in innate immune response to bacterial infection. Neutrophils express high levels of peptidylarginine deiminase 4 (PAD4), which catalyzes histone citrullination. However, whether PAD4 or histone citrullination plays a role in chromatin structure in neutrophils is unclear. In this study, we show that the hypercitrullination of histones by PAD4 mediates chromatin decondensation. Histone hypercitrullination is detected on highly decondensed chromatin in HL-60 granulocytes and blood neutrophils. The inhibition of PAD4 decreases histone hypercitrullination and the formation of NET-like structures, whereas PAD4 treatment of HL-60 cells facilitates these processes. The loss of heterochromatin and multilobular nuclear structures is detected in HL-60 granulocytes after PAD4 activation. Importantly, citrullination of biochemically defined avian nucleosome arrays inhibits their compaction by the linker histone H5 to form higher order chromatin structures. Together, these results suggest that histone hypercitrullination has important functions in chromatin decondensation in granulocytes/neutrophils.

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Proper geometric and topological organization of DNA is essential for all chromosomal processes. Two classes of proteins play major roles in organizing chromosomes: condensin complexes and type II topoisomerases. In Escherichia coli, MukB, a structural maintenance of chromosome-like component of the bacterial condensin, and topoisomerase IV (Topo IV), a type II topoisomerase that decatenates the newly replicated daughter chromosomes, are both essential for chromosome segregation in rapidly growing cells. However, little is known about the interplay between MukB and Topo IV. Here we demonstrate a physical and functional interaction between MukB and ParC, a subunit of Topo IV, in vitro. The site of MukB interaction was located on the C-terminal domain of ParC and a loss-of-interaction mutant, ParC R705E R729A, was isolated. This variant retained full activity as a topoisomerase when reconstituted with ParE to form Topo IV. We show that MukB stimulates the superhelical DNA relaxation activity of wild-type Topo IV, but not that of Topo IV reconstituted with ParC R705E R729A.

Intrinsically disordered proteins (IDPs) play important roles in many biological systems. Given the vast conformational space that IDPs can explore, the thermodynamics of the interactions with their partners is closely linked to their biological functions. Intrinsically disordered regions of Phe-Gly nucleoporins (FG Nups) that contain multiple phenylalanineglycine repeats are of particular interest, as their interactions with transport factors (TFs) underlie the paradoxically rapid yet also highly selective transport of macromolecules mediated by the nuclear pore complex (NPC). Here, we used NMR and isothermal titration calorimetry (ITC) to thermodynamically characterize these multivalent interactions. These analyses revealed that a combination of low per-FG motif affinity and the enthalpy-entropy balance prevents highavidity interaction between FG Nups and TFs, while the large number of FG motifs promotes frequent FG-TF contacts, resulting in enhanced selectivity.Our thermodynamic model underlines the importance of functional disorder of FG Nups. It helps explain the rapid and selective translocation of TFs through the NPC and further expands our understanding of the mechanisms of "fuzzy" interactions involving IDPs.Intrinsically disordered proteins (IDPs) and proteins with intrinsically disordered regions (IDRs), constitute ~30-40% of the human proteome and are involved in many protein signaling and regulation processes (1). IDPs/IDRs can interact with their targets with high specificity, and yet often with low affinity and high reversibility. There is a broad interest in quantifying the thermodynamic driving forces governing IDP interactions. Many IDPs undergo a disorder-to-order transition upon binding to their targets (2), while others form 'fuzzy complexes' (3) where significant residual disorder is maintained in the interacting state. Due to their essential role in many biological processes, a better understanding of the energetics of IDP interactions is needed (4).Many IDP interactions are mediated by short linear motifs (SLiMs) that engage with receptor molecules. Because SLiMs do not have extensive interaction interfaces to induce high enthalpy, SLiM-containing IDPs often utilize multiple motifs to participate in multivalent interactions Thermodynamics of FG-Transport Factor Interaction2 enhances individually weak monovalent interactions, resulting in higher overall affinity (avidity) and specificity (6,7). One example of an IDR that utilizes multiple short linear motifs are disordered domains of Phe-Gly nucleoporins (FG Nups) which line the central channel of the nuclear pore complex (NPC) ( Figure 1A). FG Nups typically contain 5-50 FG motifs separated by spacer residues (8). These FG repeat regions collectively form a selectively permeable barrier for macromolecular transport through the NPC. Specific cargoes can translocate rapidly and efficiently through the NPC by binding to cognate transport factors (TFs). TFs make contacts with multiple FG repeat motifs, allowing them to diffuse rapidl...

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Summary The cellular function of Escherichia coli topoisomerase III remains elusive. We show that rescue of temperature-sensitive mutants in parE and parC (encoding the subunits of the chromosomal decatenase topoisomerase IV) at restrictive temperatures by high-copy suppressors is strictly dependent on topB (encoding topoisomerase III). Double mutants of parEΔtopB and parCΔtopB were barely viable, grew slowly, and were defective in chromosome segregation at permissive temperatures. The topB mutant phenotype did not result from accumulation of toxic recombination intermediates, because it was not relieved by mutations in either recQ or recA. In addition, in an otherwise wild-type genetic background, ΔtopB cells treated with the type II topoisomerase inhibitor novobiocin displayed aberrant chromosome segregation. This novobiocin sensitivity was attributable to an increased demand for topoisomerase IV and is unlikely to define a new role for topoiosmerase III; therefore, these results suggest that topoisomerase III participates in orderly and efficient chromosome segregation in E. coli.

The study of the nuclear pore complex (NPC) is a fascinating endeavor, as it not only implies uncovering the ‘engineering marvel’ of its architecture and function, but also provides a key window into a significant evolutionary event: the origin of the eukaryotic cell. The combined efforts of many groups in the field, with the help of novel methodologies and new model organisms, are facilitating a much deeper understanding of this complex assembly. Here we cover recent advances on the characterization of the structure of the NPC scaffold and of the biophysical mechanisms that define the permeability barrier. We identify common architectural and functional principles between those two NPC compartments, expanding the previous protocoatomer hypothesis to suggest possible evolutionary origins for the FG nucleoporins and the NPC permeability barrier.

Edited by Patrick SungProperly condensed chromosomes are necessary for accurate segregation of the sisters after DNA replication. The Escherichia coli condesin is MukB, a structural maintenance of chromosomes (SMC)-like protein, which forms a complex with MukE and the kleisin MukF. MukB is known to be able to mediate knotting of a DNA ring, an intramolecular reaction. In our investigations of how MukB condenses DNA we discovered that it can also mediate catenation of two DNA rings, an intermolecular reaction. This activity of MukB requires DNA binding by the head domains of the protein but does not require either ATP or its partner proteins MukE or MukF. The ability of MukB to mediate DNA catenation underscores its potential for bringing distal regions of a chromosome together.The Escherichia coli chromosome is condensed about 1000-fold to fit into the nucleoid of the bacterium. A number of factors contribute to this extreme DNA condensation, among them are: DNA supercoiling, the binding of various nucleoid associated proteins like HU, Fis, and H-NS, and the binding of the structural maintenance of chromosomes (SMC) 3 -like condensin MukBEF (1, 2). SMC proteins act to manage the shape and behavior of chromosomes in both prokaryotes and eukaryotes (3). These proteins dimerize at a hinge region that is flanked by long coiled-coil regions that can be 40 -50 nm in length and that end in head domains that bind and hydrolyze ATP, as well as the bridging kleisin protein (MukF for the E. coli condensin (4)). The kleisin is then itself bound by another protein (MukE for the E. coli condensin (4)). There are three versions of the eukaryotic SMC proteins: the condensin, required for packaging of the chromosomes and comprised of SMC2 and SMC4; the cohesin, required for holding sister chromosomes together during mitosis and comprised of SMC1 and SMC3; and the SMC5-SMC6 complex, required for various aspects of DNA repair (3).It is generally acknowledged that the eukaryotic condensin and cohesin trap DNA topologically in the protein triangle formed by the SMC proteins and the kleisin (5, 6), although an alternative model for DNA binding for the eukaryotic cohesin exists (7). Recent reports suggest that the Bacillus subtilis SMC protein (8) and MukB (9) also trap chromosomes topologically. MukB binds linear and circular DNA in vitro and can induce negative supercoils and knots in relaxed circular DNA in the presence of a topoisomerase (10). Binding of MukB to chromosomal DNA in vivo requires ATP and MukEF (11, 12), although MukB DNA binding in vitro does not (10, 13).We (14) and the Burger and Oakley (15) labs have shown that the MukB hinge region interacts with the C-terminal ␤-propeller region of the ParC subunit of the cellular decatenase topoisomerase IV (Topo IV). We have reported (16) that this interaction stimulates the intramolecular activities of Topo IV, negative supercoil relaxation and knotting, but not the intermolecular activities of Topo IV, catenation/decatenation of DNA rings; whereas Berger, Oakley and colleag...