Exploiting bacterial effector proteins to uncover evolutionarily conserved antiviral host machinery.

Arboviruses are a diverse group of insect-transmitted pathogens that pose global public health challenges. Identifying evolutionarily conserved host factors that combat arbovirus replication in disparate eukaryotic hosts is important as they may tip the balance between productive and abortive viral replication, and thus determine virus host range. Here, we exploit naturally abortive arbovirus infections that we identified in lepidopteran cells and use bacterial effector proteins to uncover host factors restricting arbovirus replication. Bacterial effectors are proteins secreted by pathogenic bacteria into eukaryotic hosts cells that can inhibit antimicrobial defenses. Since bacteria and viruses can encounter common host defenses, we hypothesized that some bacterial effectors may inhibit host factors that restrict arbovirus replication in lepidopteran cells. Thus, we used bacterial effectors as molecular tools to identify host factors that restrict four distinct arboviruses in lepidopteran cells. By screening 210 effectors encoded by seven different bacterial pathogens, we identify several effectors that individually rescue the replication of all four arboviruses. We show that these effectors encode diverse enzymatic activities that are required to break arbovirus restriction. We further characterize Shigella flexneri-encoded IpaH4 as an E3 ubiquitin ligase that directly ubiquitinates two evolutionarily conserved proteins, SHOC2 and PSMC1, promoting their degradation in insect and human cells. We show that depletion of either SHOC2 or PSMC1 in insect or human cells promotes arbovirus replication, indicating that these are ancient virus restriction factors conserved across invertebrate and vertebrate hosts. Collectively, our study reveals a novel pathogen-guided approach to identify conserved antimicrobial machinery, new effector functions, and conserved roles for SHOC2 and PSMC1 in virus restriction.

Biochemical and structural insights into a 5' to 3' RNA ligase reveal a potential role in tRNA ligation.

ATP-grasp superfamily enzymes contain a hand-like ATP-binding fold and catalyze a variety of reactions using a similar catalytic mechanism. More than 30 protein families are categorized in this superfamily, and they are involved in a plethora of cellular processes and human diseases. Here we identify C12orf29 as an atypical ATP-grasp enzyme that ligates RNA. Human C12orf29 and its homologs auto-adenylate on an active site Lys residue as part of a reaction intermediate that specifically ligates RNA halves containing a 5'-phosphate and a 3'-hydroxyl. C12orf29 binds tRNA in cells and can ligate tRNA within the anticodon loop in vitro. Genetic depletion of c12orf29 in female mice alters global tRNA levels in brain. Furthermore, crystal structures of a C12orf29 homolog from Yasminevirus bound to nucleotides reveal a minimal and atypical RNA ligase fold with a unique active site architecture that participates in catalysis. Collectively, our results identify C12orf29 as an RNA ligase and suggest its involvement in tRNA biology.

A domain-swapped CaMKII conformation facilitates linker-mediated allosteric regulation.

Ca2+ signaling plays a key role in physiological processes such as memory formation and cardiac function. Ca2+/calmodulin-dependent protein kinase II (CaMKII) is the primary kinase that responds to Ca2+ inputs in these cells. There are four CaMKII paralogs in mammals which are alternatively spliced in the variable linker region to create upwards of 70 different variants. In this study, we systematically studied different linker regions and determined that the position of charged residues within the linker region modulates the Ca2+/CaM sensitivity of the holoenzyme. We present an X-ray crystal structure of full-length CaMKIIδ that shows a domain-swapped conformation of the subunits within the dodecameric holoenzyme. In this structure, the kinase domain of one subunit is docked onto the hub domain of a different subunit, providing an additional interface within the holoenzyme. Mutations at the equatorial and lateral interfaces revealed that the kinase-hub interaction dissociates as the hub-hub interfaces are disturbed, which led alterations in the stoichiometry of CaMKII holoenzyme and Ca2+/CaM sensitivity. Molecular dynamics simulations of linker-containing domain-swapped and non-domain-swapped CaMKIIs reveal that the domain-swapped configuration facilitates an interaction between the calmodulin binding domain and the variable linker region, such that dynamic electrostatic forces between charges on these segments can modulate the equilibrium between the compact and extended conformational states of the holoenzyme. Small angle X-ray scattering data confirms that a negatively charged linker CaMKII holoenzyme adopts a more compact conformation compared to a positively charged linker. These data support a model where patches of charged linker residues interact with the calmodulin binding domain to allosterically regulate sensitivity to Ca2+/CaM. Our findings provide a new framework for understanding CaMKII structure and allosteric regulation by the variable linker region in Ca2+-sensitive cells.

Exploiting Bacterial Effector Proteins to Uncover Evolutionarily Conserved Antiviral Host Machinery.

Arboviruses are a diverse group of insect-transmitted pathogens that pose global public health challenges. Identifying evolutionarily conserved host factors that combat arbovirus replication in disparate eukaryotic hosts is important as they may tip the balance between productive and abortive viral replication, and thus determine virus host range. Here, we exploit naturally abortive arbovirus infections that we identified in lepidopteran cells and use bacterial effector proteins to uncover host factors restricting arbovirus replication. Bacterial effectors are proteins secreted by pathogenic bacteria into eukaryotic hosts cells that can inhibit antimicrobial defenses. Since bacteria and viruses can encounter common host defenses, we hypothesized that some bacterial effectors may inhibit host factors that restrict arbovirus replication in lepidopteran cells. Thus, we used bacterial effectors as molecular tools to identify host factors that restrict four distinct arboviruses in lepidopteran cells. By screening 210 effectors encoded by seven different bacterial pathogens, we identify six effectors that individually rescue the replication of all four arboviruses. We show that these effectors encode diverse enzymatic activities that are required to break arbovirus restriction. We further characterize Shigella flexneri-encoded IpaH4 as an E3 ubiquitin ligase that directly ubiquitinates two evolutionarily conserved proteins, SHOC2 and PSMC1, promoting their degradation in insect and human cells. We show that depletion of either SHOC2 or PSMC1 in insect or human cells promotes arbovirus replication, indicating that these are ancient virus restriction factors conserved across invertebrate and vertebrate hosts. Collectively, our study reveals a novel pathogen-guided approach to identify conserved antimicrobial machinery, new effector functions, and conserved roles for SHOC2 and PSMC1 in virus restriction.

Pseudomonas effector AvrB is a glycosyltransferase that rhamnosylates plant guardee protein RIN4.

The plant pathogen Pseudomonas syringae encodes a type III secretion system avirulence effector protein, AvrB, that induces a form of programmed cell death called the hypersensitive response in plants as a defense mechanism against systemic infection. Despite the well-documented catalytic activities observed in other Fido (Fic, Doc, and AvrB) proteins, the enzymatic activity and target substrates of AvrB have remained elusive. Here, we show that AvrB is an unprecedented glycosyltransferase that transfers rhamnose from UDP-rhamnose to a threonine residue of the Arabidopsis guardee protein RIN4. We report structures of various enzymatic states of the AvrB-catalyzed rhamnosylation reaction of RIN4, which reveal the structural and mechanistic basis for rhamnosylation by a Fido protein. Collectively, our results uncover an unexpected reaction performed by a prototypical member of the Fido superfamily while providing important insights into the plant hypersensitive response pathway and foreshadowing more diverse chemistry used by Fido proteins and their substrates.

Structural insights for selective disruption of Beclin 1 binding to Bcl-2.

Stimulation of autophagy could provide powerful therapies for multiple diseases, including cancer and neurodegeneration. An attractive drug target for this purpose is Bcl-2, which inhibits autophagy by binding to the Beclin 1 BH3-domain. However, compounds that preclude Beclin 1/Bcl-2 binding might also induce apoptosis, which is inhibited by binding of Bcl-2 to BH3-domains of pro-apoptosis factors such as Bax. Here we describe the NMR structure of Bcl-2 bound to 35, a compound that we recently found to inhibit Beclin 1/Bcl-2 binding more potently than Bax/Bcl-2 binding. The structure shows that 35 binds at one end of the BH3-binding groove of Bcl-2. Interestingly, much of the 35-binding site is not involved in binding to Bcl-2 inhibitors described previously and mediates binding to Beclin 1 but not Bax. The structure suggests potential avenues to design compounds that disrupt Beclin 1/Bcl-2 binding and stimulate autophagy without inducing apoptosis.

Molecular determinants for differential activation of the bile acid receptor from the pathogen Vibrio parahaemolyticus.

Bile acids are important for digestion of food and antimicrobial activity. Pathogenic Vibrio parahaemolyticus senses bile acids and induce pathogenesis. The bile acid taurodeoxycholate (TDC) was shown to activate the master regulator, VtrB, of this system, whereas other bile acids such as chenodeoxycholate (CDC) do not. Previously, VtrA-VtrC was discovered to be the co-component signal transduction system that binds bile acids and induces pathogenesis. TDC binds to the periplasmic domain of the VtrA-VtrC complex, activating a DNA-binding domain in VtrA that then activates VtrB. Here, we find that CDC and TDC compete for binding to the VtrA-VtrC periplasmic heterodimer. Our crystal structure of the VtrA-VtrC heterodimer bound to CDC revealed CDC binds in the same hydrophobic pocket as TDC but differently. Using isothermal titration calorimetry, we observed that most mutants in the binding pocket of VtrA-VtrC caused a decrease in bile acid binding affinity. Notably, two mutants in VtrC bound bile acids with a similar affinity as the WT protein but were attenuated for TDC-induced type III secretion system 2 activation. Collectively, these studies provide a molecular explanation for the selective pathogenic signaling by V. parahaemolyticus and reveal insight into a host's susceptibility to disease.

Repurposed dihydroorotate dehydrogenase inhibitors with efficacy against drug-resistant Acinetobacter baumannii.

New antimicrobials are needed for the treatment of extensively drug-resistant Acinetobacter baumannii. The de novo pyrimidine biosynthetic enzyme dihydroorotate dehydrogenase (DHODH) is a validated drug target for malaria and human autoimmune diseases. We provide genetic evidence that A. baumannii DHODH (AbDHODH) is essential for bacterial survival in rodent infection models. We chemically validate the target by repurposing a unique library of ~450 triazolopyrimidine/imidazopyrimidine analogs developed for our malaria DHODH program to identify 21 compounds with submicromolar activity on AbDHODH. The most potent (DSM186, DHODH IC50 28 nM) had a minimal inhibitory concentration of ≤1 µg/ml against geographically diverse A. baumannii strains, including meropenem-resistant isolates. A structurally related analog (DSM161) with a long in vivo half-life conferred significant protection in the neutropenic mouse thigh infection model. Encouragingly, the development of resistance to these compounds was not identified in vitro or in vivo. Lastly, the X-ray structure of AbDHODH bound to DSM186 was solved to 1.4 Å resolution. These data support the potential of AbDHODH as a drug target for the development of antimicrobials for the treatment of A. baumannii and potentially other high-risk bacterial infections.

Structure and dynamics of major histocompatibility class Ib molecule H2-M3 complexed with mitochondrial-derived peptides.

Presentation of antigenic peptides to T-cell receptors is an essential step in the adaptive immune response. In the mouse the class Ib major histocompatibility complex molecule, H2-M3, presents bacterial- and mitochondrial-derived peptides to T-cell receptors on cytotoxic T cells. Four mitochondrial heptapeptides, differing only at residue 6, form complexes with H2-M3 which can be distinguished by T cells. No structures of relevant receptors are available. To investigate the structural basis for this distinction, crystal structures were determined and molecular dynamics simulations over one microsecond were done for each complex. In the crystal structures of the heptapeptide complexes with H2-M3, presented here, the side chains of the peptide residues at position 6 all point into the H2-M3 binding groove, and are thus inaccessible, so that the very similar structures do not suggest how recognition and initiation of responses by the T cells may occur. However, conformational differences, which could be crucial to T-cell discrimination, appear within one microsecond during molecular dynamics simulations of the four complexes. Specifically, the three C-terminal residues of peptide ligands with alanine or threonine at position 6 partially exit the binding groove; this does not occur in peptide ligands with isoleucine or valine at position 6. Structural changes associated with partial peptide exit from the binding groove, along with relevant peptide binding energetics and immunological results are discussed. Communicated by Ramaswamy H. Sarma.

Dynamic remodeling of host membranes by self-organizing bacterial effectors.

During infection, intracellular bacterial pathogens translocate a variety of effectors into host cells that modify host membrane trafficking for their benefit. We found a self-organizing system consisting of a bacterial phosphoinositide kinase and its opposing phosphatase that formed spatiotemporal patterns, including traveling waves, to remodel host cellular membranes. The Legionella effector MavQ, a phosphatidylinositol (PI) 3-kinase, was targeted to the endoplasmic reticulum (ER). MavQ and the Legionella PI 3-phosphatase SidP, even in the absence of other bacterial components, drove rapid PI 3-phosphate turnover on the ER and spontaneously formed traveling waves that spread along ER subdomains inducing vesicle and tubule budding. Thus, bacteria can exploit a self-organizing membrane-targeting mechanism to hijack host cellular structures for survival.

Potent Antimalarials with Development Potential Identified by Structure-Guided Computational Optimization of a Pyrrole-Based Dihydroorotate Dehydrogenase Inhibitor Series.

Dihydroorotate dehydrogenase (DHODH) has been clinically validated as a target for the development of new antimalarials. Experience with clinical candidate triazolopyrimidine DSM265 (1) suggested that DHODH inhibitors have great potential for use in prophylaxis, which represents an unmet need in the malaria drug discovery portfolio for endemic countries, particularly in areas of high transmission in Africa. We describe a structure-based computationally driven lead optimization program of a pyrrole-based series of DHODH inhibitors, leading to the discovery of two candidates for potential advancement to preclinical development. These compounds have improved physicochemical properties over prior series frontrunners and they show no time-dependent CYP inhibition, characteristic of earlier compounds. Frontrunners have potent antimalarial activity in vitro against blood and liver schizont stages and show good efficacy in Plasmodium falciparum SCID mouse models. They are equally active against P. falciparum and Plasmodium vivax field isolates and are selective for Plasmodium DHODHs versus mammalian enzymes.

Alternative pathways utilize or circumvent putrescine for biosynthesis of putrescine-containing rhizoferrin.

The siderophore rhizoferrin (N1,N4-dicitrylputrescine) is produced in fungi and bacteria to scavenge iron. Putrescine-producing bacterium Ralstonia pickettii synthesizes rhizoferrin and encodes a single nonribosomal peptide synthetase-independent siderophore (NIS) synthetase. From biosynthetic logic, we hypothesized that this single enzyme is sufficient for rhizoferrin biosynthesis. We confirmed this by expression of R. pickettii NIS synthetase in Escherichia coli, resulting in rhizoferrin production. This was further confirmed in vitro using the recombinant NIS synthetase, synthesizing rhizoferrin from putrescine and citrate. Heterologous expression of homologous lbtA from Legionella pneumophila, required for rhizoferrin biosynthesis in that species, produced siderophore activity in E. coli. Rhizoferrin is also synthesized by Francisella tularensis and Francisella novicida, but unlike R. pickettii or L. pneumophila, Francisella species lack putrescine biosynthetic pathways because of genomic decay. Francisella encodes a NIS synthetase FslA/FigA and an ornithine decarboxylase homolog FslC/FigC, required for rhizoferrin biosynthesis. Ornithine decarboxylase produces putrescine from ornithine, but we show here in vitro that FigA synthesizes N-citrylornithine, and FigC is an N-citrylornithine decarboxylase that together synthesize rhizoferrin without using putrescine. We co-expressed F. novicida figA and figC in E. coli and produced rhizoferrin. A 2.1 Å X-ray crystal structure of the FigC N-citrylornithine decarboxylase reveals how the larger substrate is accommodated and how active site residues have changed to recognize N-citrylornithine. FigC belongs to a new subfamily of alanine racemase-fold PLP-dependent decarboxylases that are not involved in polyamine biosynthesis. These data reveal a natural product biosynthetic workaround that evolved to bypass a missing precursor and re-establish it in the final structure.

A distinct inhibitory mechanism of the V-ATPase by Vibrio VopQ revealed by cryo-EM.

The Vibrio parahaemolyticus T3SS effector VopQ targets host-cell V-ATPase, resulting in blockage of autophagic flux and neutralization of acidic compartments. Here, we report the cryo-EM structure of VopQ bound to the Vo subcomplex of the V-ATPase. VopQ inserts into membranes and forms an unconventional pore while binding directly to subunit c of the V-ATPase membrane-embedded subcomplex Vo. We show that VopQ arrests yeast growth in vivo by targeting the immature Vo subcomplex in the endoplasmic reticulum (ER), thus providing insight into the observation that VopQ kills cells in the absence of a functional V-ATPase. VopQ is a bacterial effector that has been discovered to inhibit a host-membrane megadalton complex by coincidentally binding its target, inserting into a membrane and disrupting membrane potential. Collectively, our results reveal a mechanism by which bacterial effectors modulate host cell biology and provide an invaluable tool for future studies on V-ATPase-mediated membrane fusion and autophagy.

Lead Optimization of a Pyrrole-Based Dihydroorotate Dehydrogenase Inhibitor Series for the Treatment of Malaria.

Malaria puts at risk nearly half the world's population and causes high mortality in sub-Saharan Africa, while drug resistance threatens current therapies. The pyrimidine biosynthetic enzyme dihydroorotate dehydrogenase (DHODH) is a validated target for malaria treatment based on our finding that triazolopyrimidine DSM265 (1) showed efficacy in clinical studies. Herein, we describe optimization of a pyrrole-based series identified using a target-based DHODH screen. Compounds with nanomolar potency versus Plasmodium DHODH and Plasmodium parasites were identified with good pharmacological properties. X-ray studies showed that the pyrroles bind an alternative enzyme conformation from 1 leading to improved species selectivity versus mammalian enzymes and equivalent activity on Plasmodium falciparum and Plasmodium vivax DHODH. The best lead DSM502 (37) showed in vivo efficacy at similar levels of blood exposure to 1, although metabolic stability was reduced. Overall, the pyrrole-based DHODH inhibitors provide an attractive alternative scaffold for the development of new antimalarial compounds.

A Legionella effector kinase is activated by host inositol hexakisphosphate.

The transfer of a phosphate from ATP to a protein substrate, a modification known as protein phosphorylation, is catalyzed by protein kinases. Protein kinases play a crucial role in virtually every cellular activity. Recent studies of atypical protein kinases have highlighted the structural similarity of the kinase superfamily despite notable differences in primary amino acid sequence. Here, using a bioinformatics screen, we searched for putative protein kinases in the intracellular bacterial pathogen Legionella pneumophila and identified the type 4 secretion system effector Lpg2603 as a remote member of the protein kinase superfamily. Employing an array of biochemical and structural biology approaches, including in vitro kinase assays and isothermal titration calorimetry, we show that Lpg2603 is an active protein kinase with several atypical structural features. Importantly, we found that the eukaryote-specific host signaling molecule inositol hexakisphosphate (IP6) is required for Lpg2603 kinase activity. Crystal structures of Lpg2603 in the apo-form and when bound to IP6 revealed an active-site rearrangement that allows for ATP binding and catalysis. Our results on the structure and activity of Lpg2603 reveal a unique mode of regulation of a protein kinase, provide the first example of a bacterial kinase that requires IP6 for its activation, and may aid future work on the function of this effector during Legionella pathogenesis.

Genetic and structural studies of RABL3 reveal an essential role in lymphoid development and function.

The small GTPase RABL3 is an oncogene of unknown physiological function. Homozygous knockout alleles of mouse Rabl3 were embryonic lethal, but a viable hypomorphic allele (xiamen [xm]) causing in-frame deletion of four amino acids from the interswitch region resulted in profound defects in lymphopoiesis. Impaired lymphoid progenitor development led to deficiencies of B cells, T cells, and natural killer (NK) cells in Rabl3xm/xm mice. T cells and NK cells exhibited impaired cytolytic activity, and mice infected with mouse cytomegalovirus (MCMV) displayed elevated titers in the spleen. Myeloid cells were normal in number and function. Biophysical and crystallographic studies demonstrated that RABL3 formed a homodimer in solution via interactions between the effector binding surfaces on each subunit; monomers adopted a typical small G protein fold. RABL3xm displayed a large compensatory alteration in switch I, which adopted a ß-strand configuration normally provided by the deleted interswitch residues, thereby permitting homodimer formation. Dysregulated effector binding due to conformational changes in the switch I-interswitch-switch II module likely underlies the xm phenotype. One such effector may be GPR89, putatively an ion channel or G protein-coupled receptor (GPCR). RABL3, but not RABL3xm, strongly associated with and stabilized GPR89, and an N-ethyl-N-nitrosourea (ENU)-induced mutation (explorer) in Gpr89 phenocopied Rabl3xm.

A Bacterial Effector Mimics a Host HSP90 Client to Undermine Immunity.

The molecular chaperone HSP90 facilitates the folding of several client proteins, including innate immune receptors and protein kinases. HSP90 is an essential component of plant and animal immunity, yet pathogenic strategies that directly target the chaperone have not been described. Here, we identify the HopBF1 family of bacterial effectors as eukaryotic-specific HSP90 protein kinases. HopBF1 adopts a minimal protein kinase fold that is recognized by HSP90 as a host client. As a result, HopBF1 phosphorylates HSP90 to completely inhibit the chaperone's ATPase activity. We demonstrate that phosphorylation of HSP90 prevents activation of immune receptors that trigger the hypersensitive response in plants. Consequently, HopBF1-dependent phosphorylation of HSP90 is sufficient to induce severe disease symptoms in plants infected with the bacterial pathogen, Pseudomonas syringae. Collectively, our results uncover a family of bacterial effector kinases with toxin-like properties and reveal a previously unrecognized betrayal mechanism by which bacterial pathogens modulate host immunity.

High-resolution cryo-EM structures of the E. coli hemolysin ClyA oligomers.

Pore-forming proteins (PFPs) represent a functionally important protein family, that are found in organisms from viruses to humans. As a major branch of PFPs, bacteria pore-forming toxins (PFTs) permeabilize membranes and usually cause the death of target cells. E. coli hemolysin ClyA is the first member with the pore complex structure solved among α-PFTs, employing α-helices as transmembrane elements. ClyA is proposed to form pores composed of various numbers of protomers. With high-resolution cryo-EM structures, we observe that ClyA pore complexes can exist as newly confirmed oligomers of a tridecamer and a tetradecamer, at estimated resolutions of 3.2 Å and 4.3 Å, respectively. The 2.8 Å cryo-EM structure of a dodecamer dramatically improves the existing structural model. Structural analysis indicates that protomers from distinct oligomers resemble each other and neighboring protomers adopt a conserved interaction mode. We also show a stabilized intermediate state of ClyA during the transition process from soluble monomers to pore complexes. Unexpectedly, even without the formation of mature pore complexes, ClyA can permeabilize membranes and allow leakage of particles less than ~400 Daltons. In addition, we are the first to show that ClyA forms pore complexes in the presence of cholesterol within artificial liposomes. These findings provide new mechanistic insights into the dynamic process of pore assembly for the prototypical α-PFT ClyA.

Bacterial pseudokinase catalyzes protein polyglutamylation to inhibit the SidE-family ubiquitin ligases.

Enzymes with a protein kinase fold transfer phosphate from adenosine 5'-triphosphate (ATP) to substrates in a process known as phosphorylation. Here, we show that the Legionella meta-effector SidJ adopts a protein kinase fold, yet unexpectedly catalyzes protein polyglutamylation. SidJ is activated by host-cell calmodulin to polyglutamylate the SidE family of ubiquitin (Ub) ligases. Crystal structures of the SidJ-calmodulin complex reveal a protein kinase fold that catalyzes ATP-dependent isopeptide bond formation between the amino group of free glutamate and the γ-carboxyl group of an active-site glutamate in SidE. We show that SidJ polyglutamylation of SidE, and the consequent inactivation of Ub ligase activity, is required for successful Legionella replication in a viable eukaryotic host cell.

Importin-9 wraps around the H2A-H2B core to act as nuclear importer and histone chaperone.

We report the crystal structure of nuclear import receptor Importin-9 bound to its cargo, the histones H2A-H2B. Importin-9 wraps around the core, globular region of H2A-H2B to form an extensive interface. The nature of this interface coupled with quantitative analysis of deletion mutants of H2A-H2B suggests that the NLS-like sequences in the H2A-H2B tails play a minor role in import. Importin-9•H2A-H2B is reminiscent of interactions between histones and histone chaperones in that it precludes H2A-H2B interactions with DNA and H3-H4 as seen in the nucleosome. Like many histone chaperones, which prevent inappropriate non-nucleosomal interactions, Importin-9 also sequesters H2A-H2B from DNA. Importin-9 appears to act as a storage chaperone for H2A-H2B while escorting it to the nucleus. Surprisingly, RanGTP does not dissociate Importin-9•H2A-H2B but assembles into a RanGTP•Importin-9•H2A-H2B complex. The presence of Ran in the complex, however, modulates Imp9-H2A-H2B interactions to facilitate its dissociation by DNA and assembly into a nucleosome.