Cytokinetic abscission in Toxoplasma gondii is governed by protein phosphatase 2A and the daughter cell scaffold complex.

Cytokinetic abscission marks the final stage of cell division, during which the daughter cells physically separate through the generation of new barriers, such as the plasma membrane or cell wall. While the contractile ring plays a central role during cytokinesis in bacteria, fungi and animal cells, the process diverges in Apicomplexa. In Toxoplasma gondii, two daughter cells are formed within the mother cell by endodyogeny. The mechanism by which the progeny cells acquire their plasma membrane during the disassembly of the mother cell, allowing daughter cells to emerge, remains unknown. Here we identify and characterize five T. gondii proteins, including three protein phosphatase 2A subunits, which exhibit a distinct and dynamic localization pattern during parasite division. Individual downregulation of these proteins prevents the accumulation of plasma membrane at the division plane, preventing the completion of cellular abscission. Remarkably, the absence of cytokinetic abscission does not hinder the completion of subsequent division cycles. The resulting progeny are able to egress from the infected cells but fail to glide and invade, except in cases of conjoined twin parasites.

Non-Invasive Delivery of Negatively Charged Nanobodies by Anodal Iontophoresis: When Electroosmosis Dominates Electromigration.

Iontophoresis enables the non-invasive transdermal delivery of moderately-sized proteins and the needle-free cutaneous delivery of antibodies. However, simple descriptors of protein characteristics cannot accurately predict the feasibility of iontophoretic transport. This study investigated the cathodal and anodal iontophoretic transport of the negatively charged M7D12H nanobody and a series of negatively charged variants with single amino acid substitutions. Surprisingly, M7D12H and its variants were only delivered transdermally by anodal iontophoresis. In contrast, transdermal permeation after cathodal iontophoresis and passive diffusion was

Design, synthesis and modelling of photoreactive chemical probes for investigating target engagement of plasmepsin IX and X in Plasmodium falciparum.

The emergence of Plasmodium parasite resistance to current front-line antimalarial treatments poses a serious threat to global malaria control and highlights the necessity for the development of therapeutics with novel targets and mechanisms of action. Plasmepsins IX and X (PMIX/PMX) have been recognised as highly promising targets in Plasmodium due to their contribution to parasite's pathogenicity. Recent research has demonstrated that dual PMIX/PMX inhibition results in the impairment of multiple parasite's life cycle stages, which is an important feature in drug resistance prevention. Herein we report novel hydroxyethylamine photoaffinity labelling (PAL) probes, designed for PMIX/PMX target engagement and proteomics experiments in Plasmodium parasites. The prepared probes have both a photoreactive group (diazirine or benzophenone) for covalent attachment to target proteins, and a terminal alkyne handle allowing their use in bioorthogonal ligation. One of the synthesised benzophenone probes was shown to be highly promising as demonstrated by its outstanding antimalarial potency (IC50 = 15 nM versus D10 P. falciparum) and its inhibitory effect against PfPMX in an enzymatic assay. Molecular docking and molecular dynamics studies show that the inclusion of the benzophenone and alkyne handle does not alter the binding mode compared to the parent compound. The photoaffinity probe can be used in future chemical proteomics studies to allow hydroxyethylamine drug scaffold target identification and validation in Plasmodium. We expect our findings to act as a tool for future investigations on PMIX/PMX inhibition in antimalarial drug discovery.

Effective Inhibition of TDP-43 Aggregation by Native State Stabilization.

Preventing the misfolding or aggregation of transactive response DNA binding protein with 43 kDa (TDP-43) is the most actively pursued disease-modifying strategy to treat amyotrophic lateral sclerosis and other neurodegenerative diseases. In this work, we provide proof of concept that native state stabilization of TDP-43 is a viable and effective strategy for treating TDP-43 proteinopathies. Firstly, we leveraged the Cryo-EM structures of TDP-43 fibrils to design C-terminal substitutions that disrupt TDP-43 aggregation. Secondly, we showed that these substitutions (S333D/S342D) stabilize monomeric TDP-43 without altering its physiological properties. Thirdly, we demonstrated that binding native oligonucleotide ligands stabilized monomeric TDP-43 and prevented its fibrillization and phase separation in the absence of direct binding to the aggregation-prone C-terminal domain. Fourthly, we showed that the monomeric TDP-43 variant could be induced to aggregate in a controlled manner, which enabled the design and implementation of a high-throughput screening assay to identify native state stabilizers of TDP-43. Altogether, our findings demonstrate that different structural domains in TDP-43 could be exploited and targeted to develop drugs that stabilize the native state of TDP-43 and provide a platform to discover novel drugs to treat TDP-43 proteinopathies.

Architecture of the MKK6-p38α complex defines the basis of MAPK specificity and activation.

The mitogen-activated protein kinase (MAPK) p38α is a central component of signaling in inflammation and the immune response and is, therefore, an important drug target. Little is known about the molecular mechanism of its activation by double phosphorylation from MAPK kinases (MAP2Ks), because of the challenge of trapping a transient and dynamic heterokinase complex. We applied a multidisciplinary approach to generate a structural model of p38α in complex with its MAP2K, MKK6, and to understand the activation mechanism. Integrating cryo-electron microscopy with molecular dynamics simulations, hydrogen-deuterium exchange mass spectrometry, and experiments in cells, we demonstrate a dynamic, multistep phosphorylation mechanism, identify catalytically relevant interactions, and show that MAP2K-disordered amino termini determine pathway specificity. Our work captures a fundamental step of cell signaling: a kinase phosphorylating its downstream target kinase.

Inactivation mechanisms of influenza A virus under pH conditions encountered in aerosol particles as revealed by whole-virus HDX-MS.

Multiple respiratory viruses, including influenza A virus (IAV), can be transmitted via expiratory aerosol particles, and aerosol pH was recently identified as a major factor influencing airborne virus infectivity. Indoors, small exhaled aerosols undergo rapid acidification to pH ~4. IAV is known to be sensitive to mildly acidic conditions encountered within host endosomes; however, it is unknown whether the same mechanisms could mediate viral inactivation within the more acidic aerosol micro-environment. Here, we identified that transient exposure to pH 4 caused IAV inactivation by a two-stage process, with an initial sharp decline in infectious titers mainly attributed to premature attainment of the post-fusion conformation of viral protein haemagglutinin (HA). Protein changes were observed by hydrogen-deuterium exchange coupled to mass spectrometry (HDX-MS) as early as 10 s post-exposure to acidic conditions. Our HDX-MS data are in agreement with other more labor-intensive structural analysis techniques, such as X-ray crystallography, highlighting the ease and usefulness of whole-virus HDX-MS for multiplexed protein analyses, even within enveloped viruses such as IAV. Additionally, virion integrity was partially but irreversibly affected by acidic conditions, with a progressive unfolding of the internal matrix protein 1 (M1) that aligned with a more gradual decline in viral infectivity with time. In contrast, no acid-mediated changes to the genome or lipid envelope were detected. Improved understanding of respiratory virus fate within exhaled aerosols constitutes a global public health priority, and information gained here could aid the development of novel strategies to control the airborne persistence of seasonal and/or pandemic influenza in the future. IMPORTANCE It is well established that COVID-19, influenza, and many other respiratory diseases can be transmitted by the inhalation of aerosolized viruses. Many studies have shown that the survival time of these airborne viruses is limited, but it remains an open question as to what drives their infectivity loss. Here, we address this question for influenza A virus by investigating structural protein changes incurred by the virus under conditions relevant to respiratory aerosol particles. From prior work, we know that expelled aerosols can become highly acidic due to equilibration with indoor room air, and our results indicate that two viral proteins are affected by these acidic conditions at multiple sites, leading to virus inactivation. Our findings suggest that the development of air treatments to quicken the speed of aerosol acidification would be a major strategy to control infectious bioburdens in the air.

Structural and regulatory insights into the glideosome-associated connector from Toxoplasma gondii.

The phylum of Apicomplexa groups intracellular parasites that employ substrate-dependent gliding motility to invade host cells, egress from the infected cells, and cross biological barriers. The glideosome-associated connector (GAC) is a conserved protein essential to this process. GAC facilitates the association of actin filaments with surface transmembrane adhesins and the efficient transmission of the force generated by myosin translocation of actin to the cell surface substrate. Here, we present the crystal structure of Toxoplasma gondii GAC and reveal a unique, supercoiled armadillo repeat region that adopts a closed ring conformation. Characterisation of the solution properties together with membrane and F-actin binding interfaces suggests that GAC adopts several conformations from closed to open and extended. A multi-conformational model for assembly and regulation of GAC within the glideosome is proposed.

Conoid extrusion regulates glideosome assembly to control motility and invasion in Apicomplexa.

Members of Apicomplexa are defined by apical cytoskeletal structures and secretory organelles, tailored for motility, invasion and egress. Gliding is powered by actomyosin-dependent rearward translocation of apically secreted transmembrane adhesins. In the human parasite Toxoplasma gondii, the conoid, composed of tubulin fibres and preconoidal rings (PCRs), is a dynamic organelle of undefined function. Here, using ultrastructure expansion microscopy, we established that PCRs serve as a hub for glideosome components including Formin1. We also identified components of the PCRs conserved in Apicomplexa, Pcr4 and Pcr5, that contain B-box zinc-finger domains, assemble in heterodimer and are essential for the formation of the structure. The fitness conferring Pcr6 tethers the PCRs to the cone of tubulin fibres. F-actin produced by Formin1 is used by Myosin H to generate the force for conoid extrusion which directs the flux of F-actin to the pellicular space, serving as gatekeeper to control parasite motility.

Structural basis of phosphatidylinositol 3-kinase C2α function.

Phosphatidylinositol 3-kinase type 2α (PI3KC2α) is an essential member of the structurally unresolved class II PI3K family with crucial functions in lipid signaling, endocytosis, angiogenesis, viral replication, platelet formation and a role in mitosis. The molecular basis of these activities of PI3KC2α is poorly understood. Here, we report high-resolution crystal structures as well as a 4.4-Å cryogenic-electron microscopic (cryo-EM) structure of PI3KC2α in active and inactive conformations. We unravel a coincident mechanism of lipid-induced activation of PI3KC2α at membranes that involves large-scale repositioning of its Ras-binding and lipid-binding distal Phox-homology and C-C2 domains, and can serve as a model for the entire class II PI3K family. Moreover, we describe a PI3KC2α-specific helical bundle domain that underlies its scaffolding function at the mitotic spindle. Our results advance our understanding of PI3K biology and pave the way for the development of specific inhibitors of class II PI3K function with wide applications in biomedicine.

Pantothenate biosynthesis is critical for chronic infection by the neurotropic parasite Toxoplasma gondii.

Coenzyme A (CoA) is an essential molecule acting in metabolism, post-translational modification, and regulation of gene expression. While all organisms synthesize CoA, many, including humans, are unable to produce its precursor, pantothenate. Intriguingly, like most plants, fungi and bacteria, parasites of the coccidian subgroup of Apicomplexa, including the human pathogen Toxoplasma gondii, possess all the enzymes required for de novo synthesis of pantothenate. Here, the importance of CoA and pantothenate biosynthesis for the acute and chronic stages of T. gondii infection is dissected through genetic, biochemical and metabolomic approaches, revealing that CoA synthesis is essential for T. gondii tachyzoites, due to the parasite's inability to salvage CoA or intermediates of the pathway. In contrast, pantothenate synthesis is only partially active in T. gondii tachyzoites, making the parasite reliant on its uptake. However, pantothenate synthesis is crucial for the establishment of chronic infection, offering a promising target for intervention against the persistent stage of T. gondii.

PI(3,4)P2-mediated cytokinetic abscission prevents early senescence and cataract formation.

Cytokinetic membrane abscission is a spatially and temporally regulated process that requires ESCRT (endosomal sorting complexes required for transport)­dependent control of membrane remodeling at the midbody, a subcellular organelle that defines the cleavage site. Alteration of ESCRT function can lead to cataract, but the underlying mechanism and its relation to cytokinesis are unclear. We found a lens-specific cytokinetic process that required PI3K-C2α (phosphatidylinositol-4-phosphate 3-kinase catalytic subunit type 2α), its lipid product PI(3,4)P2 (phosphatidylinositol 3,4-bisphosphate), and the PI(3,4)P2­binding ESCRT-II subunit VPS36 (vacuolar protein-sorting-associated protein 36). Loss of each of these components led to impaired cytokinesis, triggering premature senescence in the lens of fish, mice, and humans. Thus, an evolutionarily conserved pathway underlies the cell type­specific control of cytokinesis that helps to prevent early onset cataract by protecting from senescence.

SUV39 SET domains mediate crosstalk of heterochromatic histone marks.

The SUV39 class of methyltransferase enzymes deposits histone H3 lysine 9 di- and trimethylation (H3K9me2/3), the hallmark of constitutive heterochromatin. How these enzymes are regulated to mark specific genomic regions as heterochromatic is poorly understood. Clr4 is the sole H3K9me2/3 methyltransferase in the fission yeast Schizosaccharomyces pombe, and recent evidence suggests that ubiquitination of lysine 14 on histone H3 (H3K14ub) plays a key role in H3K9 methylation. However, the molecular mechanism of this regulation and its role in heterochromatin formation remain to be determined. Our structure-function approach shows that the H3K14ub substrate binds specifically and tightly to the catalytic domain of Clr4, and thereby stimulates the enzyme by over 250-fold. Mutations that disrupt this mechanism lead to a loss of H3K9me2/3 and abolish heterochromatin silencing similar to clr4 deletion. Comparison with mammalian SET domain proteins suggests that the Clr4 SET domain harbors a conserved sensor for H3K14ub, which mediates licensing of heterochromatin formation.

Structure of the phosphoinositide 3-kinase (PI3K) p110γ-p101 complex reveals molecular mechanism of GPCR activation.

The class IB phosphoinositide 3-kinase (PI3K), PI3Kγ, is a master regulator of immune cell function and a promising drug target for both cancer and inflammatory diseases. Critical to PI3Kγ function is the association of the p110γ catalytic subunit to either a p101 or p84 regulatory subunit, which mediates activation by G protein-coupled receptors. Here, we report the cryo-electron microscopy structure of a heterodimeric PI3Kγ complex, p110γ-p101. This structure reveals a unique assembly of catalytic and regulatory subunits that is distinct from other class I PI3K complexes. p101 mediates activation through its Gßγ-binding domain, recruiting the heterodimer to the membrane and allowing for engagement of a secondary Gßγ-binding site in p110γ. Mutations at the p110γ-p101 and p110γ-adaptor binding domain interfaces enhanced Gßγ activation. A nanobody that specifically binds to the p101-Gßγ interface blocks activation, providing a novel tool to study and target p110γ-p101-specific signaling events in vivo.

Structural insights into an atypical secretory pathway kinase crucial for Toxoplasma gondii invasion.

Active host cell invasion by the obligate intracellular apicomplexan parasites relies on the formation of a moving junction, which connects parasite and host cell plasma membranes during entry. Invading Toxoplasma gondii tachyzoites secrete their rhoptry content and insert a complex of RON proteins on the cytoplasmic side of the host cell membrane providing an anchor to which the parasite tethers. Here we show that a rhoptry-resident kinase RON13 is a key virulence factor that plays a crucial role in host cell entry. Cryo-EM, kinase assays, phosphoproteomics and cellular analyses reveal that RON13 is a secretory pathway kinase of atypical structure that phosphorylates rhoptry proteins including the components of the RON complex. Ultimately, RON13 kinase activity controls host cell invasion by anchoring the moving junction at the parasite-host cell interface.

Naturally acquired blocking human monoclonal antibodies to Plasmodium vivax reticulocyte binding protein 2b.

Plasmodium vivax preferentially invades reticulocytes and recognition of these cells is mediated by P. vivax Reticulocyte Binding Protein 2b (PvRBP2b) binding to human Transferrin receptor 1 (TfR1) and Transferrin (Tf). Longitudinal cohort studies in Papua New Guinea, Thailand and Brazil show that PvRBP2b antibodies are correlated with protection against P. vivax infection and disease. Here, we isolate and characterize anti-PvRBP2b human monoclonal antibodies from two individuals in Cambodia with natural P. vivax infection. These antibodies bind with high affinities and map to different regions of PvRBP2b. Several human antibodies block PvRBP2b binding to reticulocytes and inhibit complex formation with human TfR1-Tf. We describe different structural mechanisms for functional inhibition, including either steric hindrance with TfR1-Tf or the reticulocyte membrane. These results show that naturally acquired human antibodies against PvRBP2b can inhibit its function which is important for P. vivax invasion.

Gßγ is a direct regulator of endogenous p101/p110γ and p84/p110γ PI3Kγ complexes in mouse neutrophils.

The PI3Kγ isoform is activated by Gi-coupled GPCRs in myeloid cells, but the extent to which the two endogenous complexes of PI3Kγ, p101/p110γ and p84/p110γ, receive direct regulation through Gßγ or indirect regulation through RAS and the sufficiency of those inputs is controversial or unclear. We generated mice with point mutations that prevent Gßγ binding to p110γ (RK552DD) or to p101 (VVKR777AAAA) and investigated the effects of these mutations in primary neutrophils and in mouse models of neutrophilic inflammation. Loss of Gßγ binding to p110γ substantially reduced the activation of both p101/p110γ and p84/p110γ in neutrophils by various GPCR agonists. Loss of Gßγ binding to p101 caused more variable effects, depending on both the agonist and cellular response, with the biggest reductions seen in PIP3 production by primary neutrophils in response to LTB4 and MIP-2 and in the migration of neutrophils during thioglycolate-induced peritonitis or MIP2-induced ear pouch inflammation. We also observed that p101VVKR777AAAA neutrophils showed enhanced p84-dependent ROS responses to fMLP and C5a, suggesting that competition may exist between p101/p110γ and p84/p110γ for Gßγ subunits downstream of GPCR activation. GPCRs did not activate p110γ in neutrophils from mice lacking both the p101 and p84 regulatory subunits, indicating that RAS binding to p110γ is insufficient to support GPCR activation in this cell type. These findings define a direct role for Gßγ subunits in activating both of the endogenous PI3Kγ complexes and indicate that the regulatory PI3Kγ subunit biases activation toward different GPCRs.

Identification of immunodominant linear epitopes from SARS-CoV-2 patient plasma.

A novel severe acute respiratory syndrome coronavirus (SARS-CoV-2) is the source of a current pandemic (COVID-19) with devastating consequences in public health and economic stability. Using a peptide array to map the antibody response of plasma from healing patients (12) and heathy patients (6), we identified three immunodominant linear epitopes, two of which correspond to key proteolytic sites on the spike protein (S1/S2 and S2') known to be critical for cellular entry. We show biochemical evidence that plasma positive for the epitope adjacent to the S1/S2 cleavage site inhibits furin-mediated proteolysis of spike.

Auxiliary domains of the HrpB bacterial DExH-box helicase shape its RNA preferences.

RNA helicases are fundamental players in RNA metabolism: they remodel RNA secondary structures and arrange ribonucleoprotein complexes. While DExH-box RNA helicases function in ribosome biogenesis and splicing in eukaryotes, information is scarce about bacterial homologs. HrpB is the only bacterial DExH-box protein whose structure is solved. Besides the catalytic core, HrpB possesses three accessory domains, conserved in all DExH-box helicases, plus a unique C-terminal extension (CTE). The function of these auxiliary domains remains unknown. Here, we characterize genetically and biochemically Pseudomonas aeruginosa HrpB homolog. We reveal that the auxiliary domains shape HrpB RNA preferences, affecting RNA species recognition and catalytic activity. We show that, among several types of RNAs, the single-stranded poly(A) and the highly structured MS2 RNA strongly stimulate HrpB ATPase activity. In addition, deleting the CTE affects only stimulation by structured RNAs like MS2 and rRNAs, while deletion of accessory domains results in gain of poly(U)-dependent activity. Finally, using hydrogen-deuterium exchange, we dissect the molecular details of HrpB interaction with poly(A) and MS2 RNAs. The catalytic core interacts with both RNAs, triggering a conformational change that reorients HrpB. Regions within the accessory domains and CTE are, instead, specifically responsive to MS2. Altogether, we demonstrate that in bacteria, like in eukaryotes, DExH-box helicase auxiliary domains are indispensable for RNA handling.

Autoregulation of Class II Alpha PI3K Activity by Its Lipid-Binding PX-C2 Domain Module.

Class II phosphoinositide 3-kinases (PI3K-C2) are large multidomain enzymes that control cellular functions ranging from membrane dynamics to cell signaling via synthesis of 3'-phosphorylated phosphoinositides. Activity of the alpha isoform (PI3K-C2α) is associated with endocytosis, angiogenesis, and glucose metabolism. How PI3K-C2α activity is controlled at sites of endocytosis remains largely enigmatic. Here we show that the lipid-binding PX-C2 module unique to class II PI3Ks autoinhibits kinase activity in solution but is essential for full enzymatic activity at PtdIns(4,5)P2-rich membranes. Using HDX-MS, we show that the PX-C2 module folds back onto the kinase domain, inhibiting its basal activity. Destabilization of this intramolecular contact increases PI3K-C2α activity in vitro and in cells, leading to accumulation of its lipid product, increased recruitment of the endocytic effector SNX9, and facilitated endocytosis. Our studies uncover a regulatory mechanism in which coincident binding of phosphoinositide substrate and cofactor selectively activate PI3K-C2α at sites of endocytosis.