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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