Biochemical Journal

Artemisinin is an effective antimalarial drug sourced from Artemisia annua, but its low and variable yields require enhancement either semi-synthetically or in-planta to meet the global demand for treatment. Though essential enzymes have been identified in the artemisinin biosynthetic pathway, including an essential Cytochrome P450 monooxygenase (CYP71AV1), there are still many unknowns. Cytochrome P450 reductase 1 (herein, AaCPR1), has been experimentally confirmed as an electron transfer partner for CYP71AV1 in its three step oxygenation of key artemisinin precursors. However, the recent discovery of a highly related CPR, herein AaCPR2, introduces the possibility that another, potentially more catalytically favourable interaction, could exist for CYP71AV1. Therefore, enzyme kinetics and differential scanning fluorimetry (DSF) were used in the characterisation of both AaCPR1 and AaCPR2 to determine the existence and source of their catalytic differences. Tested enzyme activity under cytochrome c and NADPH concentrations revealed that AaCPR1 had lower Km and higher kcat/Km values, while AaCPR2 had higher Vmax and kcat values. This suggests that AaCPR1 is more effective at reducing cytochrome c when substrate conditions are limiting, whereas AaCPR2 is more effective than AaCPR1 at reducing cytochrome c when substrate conditions are saturating. This implies a functional partitioning of the two enzymes on the basis of substrate availability. The DSF results provided deeper insight into the different protein-ligand interactions between the two enzymes. AaCPR2 reached lower maximum melting temperatures across all tested conditions, whereas AaCPR1 had higher maximum melting temperatures. Thus, AaCPR1 exhibits higher thermal stability and has a higher temperature threshold than AaCPR2. This contributes to the notion that the AaCPRs are functionally divergent also on the basis of temperature. The cumulative differences in melting behaviour between the two enzymes led to the hypothesis that AaCPR1 and AaCPR2 exhibit different domain motions that may lead to preferential catalysis for one redox partner over another. This was further supported by the prediction of a highly variable loop region between the two enzymes at the connecting domain just after the flexible hinge. If such loops are highly mobile, as predicted, then the residue differences therein could provide a bio-structural basis for the kinetic and thermal/biophysical differences observed between AaCPR1 and AaCPR2. These data support that AaCPR1 and AaCPR2 possess fundamental biophysical differences despite their high degree of relatedness. Ultimately, these differences suggest differential metabolic functions of the two enzyme in artemisinin biosynthesis and/or other important secondary metabolic processes.

Species of Bacillus bacteria including Bacillus safensis and Bacillus subtilis are finding increasing uses in biotechnology and bioremediation, thanks in part to their metabolic robustness. Malate dehydrogenase (MDH) is at the heart of central metabolism and thus a better understanding of Bacillus MDH proteins could aid in the optimization of these applications. MDH of Bacillus spp. belong to the lactate dehydrogenase (LDH)-like class of MDHs, otherwise known as the MDH3 class. Despite wide prevalence in nature among prokaryotes and archaea, this typically homotetrameric class is understudied compared to the MDH1 and MDH2 classes found in eukaryotes. We therefore recombinantly expressed and purified MDH proteins from two societally relevant Bacillus spp.-B. safensis and B. subtilis-and characterized them biophysically (via Size Exclusion Chromatography-Small Angle X-ray Scattering (SEC-SAXS) and Differential Scanning Fluorimetry (DSF)) and enzymatically (via spectroscopic activity assays). As expected based on their high sequence identity, the two MDH orthologs had similar properties in most regards, including a tetrameric structure and high susceptibility to substrate inhibition. However, we uncovered differences in conditional thermal stability, in addition to subtle differences in enzymatic activity that offer insight into the workings of LDH-like MDH. Summary statementMalate dehydrogenase (MDH) is a fundamental metabolic enzyme, from microbes to mammals, yet comparably little is known about microbial MDH, especially MDH of the tetrameric MDH3 class. We compare the biophysical and enzymatic properties of two such enzymes from the societally relevant bacterial species Bacillus subtilis and Bacillus safensis, offering useful insight with potential biotechnological implications.

RNA helicases encoded by positive-strand RNA viruses are essential for genome replication, yet the specific biological functions and mechanochemical basis underlying these functions remain poorly defined. Progress has been limited by the difficulty of resolving individual catalytic steps under single-turnover conditions, which are often experimentally inaccessible for viral enzymes. Alphaviruses replicate within membrane-bound spherules that may alter local metabolite concentrations, raising the possibility that the enzymatic properties of alphaviral proteins differ from those of viruses with greater cytosolic exposure. Here, we present a kinetic and binding analysis of full-length non-structural protein 2 (nsP2) from Chikungunya virus, a multifunctional superfamily 1B NTPase and RNA helicase. Purified nsP2 binds nucleoside triphosphates with high affinity, exhibiting equilibrium dissociation constants in the single digit micromolar range. This property enabled single-turnover, pre-steady-state, and isotope-trapping experiments that are rarely feasible for viral helicases. These analyses identified two sequential conformational-change steps required for nucleotide hydrolysis. Molecular dynamics simulations suggest tightening of the RecA1 and RecA2 domains upon ATP binding followed by compaction of the enzyme mediated by interactions between the 1B subdomain and RecA2 domain. Product inhibition patterns support random release of ADP and inorganic phosphate, with relative binding affinities indicating that ADP dissociates first. The reaction is irreversible. Although nsP2 binds RNA tightly, strand separation under single-turnover conditions is too slow to represent ATP-driven unwinding, instead likely reflecting formation of an unwinding-competent nsP2-RNA complex. Together, these findings establish a quantitative framework for nsP2 function and provide a roadmap for mechanistic studies of alphaviral helicases. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=63 SRC="FIGDIR/small/723793v1_ufig1.gif" ALT="Figure 1"> View larger version (18K): org.highwire.dtl.DTLVardef@13899a1org.highwire.dtl.DTLVardef@ee1aadorg.highwire.dtl.DTLVardef@1991e1org.highwire.dtl.DTLVardef@b877f6_HPS_FORMAT_FIGEXP M_FIG C_FIG

Multi-functionality in extant enzymes, including the ability to transform multiple substrates, is thought to arise, in part, from conformational flexibility. The hexokinase protein family represents a classic model system for investigating the relationship between substrate specificity and conformational change. Within this family, human glucokinase (hGCK) displays notable degrees of conformational heterogeneity, including an intrinsically disordered loop. The extent to which these structural features contribute to the breadth of hGCKs substrate scope is unknown. Here, we investigate the substrate specificities of extant and ancestral glucokinases that span the evolutionary emergence of conformational heterogeneity in this family. We show that extant hGCK catalyzes the ATP-dependent phosphorylation of glucose, 2-deoxyglucose, mannose, glucosamine, fructose, allose and galactose with catalytic efficiencies ranging from 6.3 x 103 M-1 sec-1 to 0.33 M-1sec-1. A glucokinase ancestor from early vertebrate evolution (vGCK), which also displays conformational heterogeneity and disorder, phosphorylates these same seven substrates with similar kcat/Km values. An antecedent, chordate glucokinase (cGCK), which displays reduced conformational heterogeneity and lacks intrinsic disorder, also transforms these same substrates, but with higher overall catalytic efficiencies and markedly lower Km values. Notably, however, the ratios of kcat/Km values for individual substrate pairs, which define specificity, are unchanged for all three enzymes. Our results demonstrate that substrate specificity is not correlated with conformational diversity in GCKs and support a model in which the differences in catalytic efficiencies of various substrates arise from differences in the ability to form the ground state enzyme-carbohydrate binary complex.

We previously found that high genome replication fitness of the hepatitis C virus (HCV) was associated with severe disease in immunocompromised patients. Elevated replication fitness was mediated by accumulation of mutations in the replication enhancing domain (ReED) within domain (D) 2 of non-structural protein (NS) 5A. NS5A is a partially unstructured phosphoprotein lacking enzymatic activity but fulfilling a key role in HCV replication due to interacting with various cellular and viral proteins. It can exist in a variety of dimeric and oligomeric conformations mediated by NS5A D1 with clinically approved NS5A inhibitors proposed to exert their antiviral function by fixing these dimers in distinct conformations. In this study, we aimed at elucidating the ReEDs mode of action. AlphaFold modelling indicated a so far unrecognized NS5A dimerization site in the ReED. Indeed, split nano luciferase assays revealed a significantly stronger NS5A dimerization of high replicator ReED variants, suggesting that high replication fitness is mediated by enforcement of NS5A self-interaction. This hypothesis was supported by the effect of low dose (1 pM) NS5A inhibitor treatment, increasing replication fitness and phenocopying the effects of ReED mutations. Furthermore, we found that HCV isolate JFH1, replicating with very high efficiency, is completely resistant to the regulatory function of the ReED. Chimeric replicons composed of ReED resistant JFH1 and the ReED sensitive isolate J6 identified NS3 helicase and NS5B polymerase as critical genetic elements mediating ReED sensitivity/resistance. Our data overall suggest that NS5A is a negative regulator of HCV replication fitness with dimerization releasing the inhibitory interaction with helicase and/or polymerase, thereby likely facilitating initiation of RNA synthesis.

O-fucosylation plays an essential role in controlling protein folding, secretion and protein-protein interactions within the extracellular space. Recently, we identified a new form of protein O-fucosylation occurring on the N-terminal Elastin Microfibril Interaction (EMI) domain of several secreted proteins, mediated by two previously uncharacterized protein O-fucosyltransferases, POFUT3 (FUT10) and POFUT4 (FUT11). As all POFUT enzymes (POFUT1-4) are highly specific for the three-dimensional (3D) structure of their substrate protein domains, we postulated that structural homologues of these domains in other proteins may also be O-fucosylated. Here, we employed iterative protein structural homology searches as a novel strategy for identifying EMI-like domains that may serve as potential substrates for POFUT3/4. We discovered that microfibrillar-associated protein 2 and 5 (MFAP2/MFAP5) contain EMI-like domains and are O-fucosylated at high stoichiometry in human tissues. Unexpectedly, we showed that only POFUT3 is both necessary and sufficient for MFAP2/MFAP5 O-fucosylation, despite POFUT4 also having strong protein-protein interactions with MFAP2/MFAP5. Finally, we determined that O-fucosylation of MFAP2/MFAP5 is required for their efficient secretion, similar to other EMI domain-containing proteins. Together, these data demonstrate the power of sensitive structural homology analysis in identifying new enzyme-substrate relationships and protein-protein interactions.

Reactive oxygen species (ROS) are central signaling molecules in many biological processes by inducing oxidative modifications of protein cysteine residues, including S-glutathionylation. Increasing evidence supports that ROS contribute to cancer progression via promoting cancer cell migration, invasion, and metastasis. Nevertheless, the protein targets of S-glutathionylation that regulate cancer cell motility remain ill-defined. In this study, we report on the redox regulation of ARHGEF7, a guanine nucleotide exchange factor highly expressed in metastatic cancer cells, that plays a major role in regulating cell migration. Our data demonstrates that ARHGEF7 is selectively glutathionylated at the highly conserved C312 residue in its PH domain, which is implicated in regulating its enzymatic activity. Breast cancer cell lines showed increased cell migration and invasion upon glutathionylation of ARHGEF7 at C312 in response to both oxidative stress and epidermal growth factor (EGF). Mechanistically, upon C312 glutathionylation, ARHGEF7 exhibited significantly enhanced binding to Rac1 and increased Rac1 recruitment to the cell membrane and lamellipodia. ARHGEF7 S-glutathionylation also increased its enzymatic rate of GDP-GTP nucleotide exchange, resulting in Rac1 activation. Consequently, ARHGEF7 C312 S-glutathionylation induced Rac1-PAK1 activation and their downstream pathways, including LIMK1 and MEK1, thereby enhancing migration and invasion. Our data reveal a new redox player in cell migration, with its potential implications for ROS-induced cancer progression.

Calpains constitute an ancient, extensive family of calcium-dependent cysteine proteases found in some bacteria and most eukaryotes. They are involved in a wide variety of developmental and cellular processes and are implicated in major human diseases, yet it remains to be seen if they have a common core function explaining their widespread and varied presence across taxa. Beyond their core CysPc catalytic domain, calpains contain diverse domain combinations and can be either cytosolic or membrane bound. Here we hypothesize a general role for both cytosolic and transmembrane calpains in cellular cytokinesis through positional anchoring and organization of microtubules (MTs). We propose that during plant cell division, the singular transmembrane calpain DEK1 localizes and organizes the array of cortical MTs from the microtubule organizing center (MTOC) to establish the location of the preprophase band and/or the site of cell plate formation according to the positional activation of DEK1 proteins in the nuclear membrane. Similarly, during cell division in animals, their calpains may be involved in setting the point of membrane invagination via their association with membrane-bound proteins. This proposition adds to the current picture of animal MTOC/centrosome function and suggests how a calcium peak during the initial cytokinetic furrowing might be transmitted. We discuss this novel mechanistic model for calpain activity in the context of data from the animal and plant literature, as well as of our novel discovery here of calpain sequences in both brown and red algal genomes. Finally, we speculate that the ancestral role of calpains in early eukaryotes, before the split into the major eukaryotic supergroups, may have been to facilitate the formation and function of MT arrays in flagella and cilia. From this origin, calpains may have developed new functions in eukaryote cell division processes by anchoring centrosomes/MTOC to set the cell division orientations that are especially important for complex multicellularity.

DNA helicases are ATP-dependent motor proteins that catalyze duplex DNA unwinding and are involved in DNA repair, recombination and replication restart. Prominent members of the non-hexameric SF1A UvrD-family helicases are E. coli UvrD, Rep, B. stearothermophilus PcrA and M. tuberculosis UvrD1. SF1A monomers are processive 3 to 5 single stranded DNA translocases, but need to be activated to become DNA helicases. One mechanism of activation is dimerization. Whereas Rep, UvrD and PcrA form non-covalent dimers, the Mtb UvrD1 helicase forms a redox-dependent covalent dimer. Dimerization of Mtb UvrD1 occurs between the same regulatory domain (2B) within each subunit stabilized by a disulfide bond formed between the same cysteine (Cys451) within each subunit. Dimerization relieves an inhibitory interaction between the 2B domain and duplex DNA within the monomer-DNA complex. We show here that Rep, UvrD and PcrA dimerize using the same 2B-2B interface. By placing a Cys residue within the 2B domains of Rep, UvrD and PcrA in the structurally equivalent position occupied by Cys451 of Mtb UvrD1, all three enzymes form redox-dependent covalent dimers that are constitutively active helicases with increased processivity compared to the non-covalent dimers. Hence, the 2B domain is a general dimerization domain for UvrD-family SF1A helicases.

Malonate is often described as an endogenous inhibitor of complex II of the electron transport chain. However, the cellular source of malonate is unclear, and current knowledge concerning its metabolism is limited to the action of a single enzyme, Acyl-CoA Synthetase Family Member 3 (ACSF3), which converts malonate to malonyl-CoA in the mitochondrial matrix. One potential route of malonate metabolism downstream of ACSF3 is its consumption by the mitochondrial fatty acid synthesis (mtFAS) pathway. However, studies examining the link between ACSF3 and mtFAS have yielded conflicting results. We developed a novel mass spectrometry approach to perform stable isotope tracing into products of mtFAS, and found that while malonate is in fact a carbon source for mtFAS, ACSF3 is not required for malonate incorporation into mtFAS products. Using this method to trace other nutrients into mtFAS, we also found evidence of acetyl-CoA carboxylase 1 (ACC1)-dependent malonate synthesis from glucose. We further show that ACC1 is required for optimal mtFAS activity, with downstream effects on oxidative phosphorylation. Together these findings establish the malonate as a regulated endogenous intermediate that supports mtFAS activity and mitochondrial oxidative function.

NKCC2, localized to the apical membrane of thick ascending limb epithelial cells, is essential for renal salt handling and systemic electrolyte homeostasis. NKCC2 undergoes extensive ubiquitylation, with the E3 protein ligase Nedd4-2 implicated as a key regulator. However, progress has been limited by challenges expressing NKCC2 in mammalian cell lines, hindering mechanistic studies of NKCC2 ubiquitylation. Therefore, the aims of this study were to develop a mammalian cell model enabling mechanistic investigations of NKCC2 ubiquitylation, including the role of Nedd4-2 and the functional consequences of site-specific modification. A tetracycline-inducible MDCKI cell line was generated expressing human NKCC2 and used to assess Nedd4-2-dependent and site-specific ubiquitylation of NKCC2 using biochemical, imaging, and functional assays. The MDCKI cell line demonstrated stable, inducible expression of full-length human NKCC2. In this cell line, mutating the ubiquitylation site at K871 increased membrane abundance and uptake activity, without altering internalization rates. Nedd4-2 co-immunoprecipitated with NKCC2, and Nedd4-2 deletion increased total, but not membrane NKCC2 abundance. In summary, ubiquitylation on NKCC2 at K871 plays a key role in controlling NKCC2 membrane localization and thus function. Although Nedd4-2 can modulate NKCC2 abundance, it is not involved in NKCC2 trafficking. We conclude that the generated cell line provides a robust model for mechanistic studies of NKCC2 and will aid studies examining posttranslational regulation of NKCC2.

Conflicting roles have been proposed for the E. coli protein RatA. Initially described as a ribosome targeting toxin, a later report pronounced it the bacterial homologue to the inner mitochondrial membrane protein Coq10. Coq10 proteins are conserved from prokaryotes to human and implicated to serve a lipid chaperone role in the biosynthesis of ubiquinone, a crucial electron carrier during aerobic respiration. We recently identified that the contradictory results published for RatA can be attributed to a mis-annotation of the gene in the reference genome. Here, we further elucidate the molecular function of RatA. We clarify that RatA is not a toxin but serves as a lipid shuttle for ubiquinone from its cytosolic biosynthesis complex to the inner membrane. Furthermore, we show that the loss of RatA results in an impaired, but not abolished electron transport chain and demonstrate broad metabolic adaptations of the cells as a consequence. Therefore, we propose to rename RatA to UbiM to reflect its function and to be in accordance with the naming convention of other ubiquinone biosynthesis proteins.

Thermal proteome profiling (TPP) and its higher-throughput derivative, the proteome integral solubility alteration (PISA) assay, measure changes in protein thermal stability upon ligand binding or other perturbations and have been widely adopted in drug discovery and biomedical research. Though the PISA workflow is straightforward, key parameters, including detergent concentration, methods for removing denatured aggregates, and temperature range selection, vary across studies and can markedly influence assay outcomes. Yet these factors have not been systematically evaluated, limiting rational experimental design and data interpretation. Here, through a combined use of TPP, PISA, tandem mass tag (TMT)-based multiplexing, and computational simulation, we systematically characterize these parameters based on the melting behavior of [~]9,000 proteins. We find that reducing detergent concentration elevates apparent Tm by 1.5-2{degrees}C proteome-wide, and aggregate removal by filtration versus centrifugation further alters measurements. We leverage these observations to optimize PISA then apply the optimized conditions to identify the aminopeptidase NPEPPS as a previously uncharacterized binding partner of angiotensin II, a key vasoactive peptide hormone in blood pressure regulation. Together, this work provides a general framework for assay design and data interpretation, and extends the utility of PISA beyond small molecules to dissecting peptide-protein interactions, an increasingly important modality in drug discovery.

Expression of the chloroplast AAA+ chaperone CLPD gene increases during senescence and drought, but its functional role in chloroplast proteostasis is poorly understood. This study provides a comprehensive analysis of Arabidopsis CLPD protein accumulation across development from early seedlings to senescence, and compares results to its homologs CLPC1,2, as well as CLPB3 and cpHSP90. The developmental consequences of complete loss of CLPD expression (clpd-1), as well as overexpression of functional CLPD or CLPD impaired in ATP hydrolysis (CLPD-TRAP), were determined in Arabidopsis. clpd-1 has accelerated seedling development while functional CLPD overexpression lines, but not CLPD-TRAP, have delayed development. To determine if CLPD is a bona fide CLP chaperone associating with the CLPPRT protease and to identify in vivo candidate substrates, we employed the CLPD-TRAP line during the vegetative and flowering (senescent) growth stages. Affinity purification of CLPD-TRAP followed by mass spectrometry showed high enrichment of the CLP protease complex, thus providing direct support for the role of CLPD in substrate delivery to the CLP protease. CLPC1,2 were also highly enriched in the CLPD-TRAP interactome, suggesting hetero-oligomerization and cooperation between the three chaperones is likely. Nine chloroplast candidate substrates were identified in the CLPD-interactomes, including: FHY2 involved in riboflavin synthesis, THI1 and THIC involved in thiamin metabolism, and four proteins of unknown function. Several of these have been previously identified as potential CLPC1 substrates; however, others appear to be specific to CLPD. CLPD acts in substrate selection within a heteromeric CLPC-CLPD hexamer, likely to make unique contributions through its divergent N-terminus.

Biological sex has systemic effects on gene expression, cell behavior, and disease etiology. Despite these widespread effects, sex as a biological variable is understudied, particularly in chronic lung diseases. In idiopathic pulmonary fibrosis (IPF), 70% of patients are male, and male patients have overall worse survival post-diagnosis. While behavioral differences between sexes might account for some of the epidemiological differences, the contribution of underlying biology is not known. In this study, we performed regional proteomic analysis via laser-captured microdissection-coupled mass spectrometry and analyzed the data for sex-biased protein expression. We discovered that even in control lung, sex differences existed in both airway and alveolar regions. Sex differences became more pronounced in diseased regions, with sex-biased expression of diverse proteins including those involved in extracellular vesicle secretion, cellular metabolism, and extracellular matrix remodeling. These data suggest that baseline sex differences in lung proteome may contribute to sex-specific susceptibility, progression, and clinical outcomes in IPF, underscoring the need for future mechanistic and clinical studies to account for sex as a biological variable.

The brain-derived neurotrophic factor (BDNF)-tropomyosin receptor kinase B (TrkB) signalling axis is a key effector of synaptic plasticity and neuroprotection. While TrkB activation is a major objective towards preventing dysfunction of the nervous system, it cannot be reached with exogenous BDNF administration given the unfavourable physiochemical properties of BDNF. In addition, BDNF also activates a tumour necrosis factor pathway by binding to the neurotrophin receptor p75. The TrkB agonist ZEB85 provides an alternative route to the selective activation of TrkB. We report here the structural basis for the interaction between human TrkB, and both ZEB85 and BDNF, and reveal that a sulfated tyrosine modification is indispensable for ZEB85 activation of TrkB signalling. Using structure-guided BDNF- and ZEB85-binding deficient TrkB mutants, we assessed their ability to sequester ligands from full-length TrkB in cultured human neurons. We found that the BDNF binding site extends into the extracellular juxtamembrane domain of TrkB but does not require the sulfotyrosine at residue 400 to activate TrkB. Together with biophysical analysis and AlphaFold modelling these results also explain how BDNF can displace ZEB85 from TrkB through an overlapping epitope. Our findings reveal unique features of TrkB, not present in the related neurotrophin receptors TrkA and TrkC, and suggest new directions to explore the role of sulfotyrosine in TrkB signalling and identify new TrkB-specific protein ligands. One Sentence SummaryInvestigation of the mechanism of action of TrkB agonist ZEB85 extends molecular understanding of TrkB activation.

Protein tyrosine kinases (PTKs) regulate cellular biochemistry by phosphorylating tyrosine residues that alter protein function; their substrate preferences define the topology of signaling cascades. Previous studies of PTKs have mapped their average preferences for amino acids surrounding phosphorylation sites, but their ability to discriminate between highly similar substrate sequences (i.e., their sensitivity to minor changes in sequence within different regions of a substrate, and the sequence-dependent nature of this sensitivity) remains poorly understood. Here, we use a genetically encoded biosensor for PTK activity to examine the influence of local sequence context on substrate specificity. Across five well-studied PTKs, we identified amino acid substitutions within consensus substrates that could enhance selectivity for one PTK over others, confer sensitivity to substrate length, or improve PTK compatibility beyond the consensus. These effects were not predicted by classical specificity maps or advanced molecular modeling tools. Using a dual-selection screen that incorporates decoy substrates, we found additional sequence-diverse substrates with unexpectedly orthogonal PTK compatibilities. Our findings show how context-specific sequence features alter PTK substrate specificity far beyond what might be expected from classical consensus models and establish an experimental framework for defining the limits of substrate overlap between closely related kinases.

Malaria is caused by Plasmodium parasites, and its clinical symptoms are a result of parasite invasion of red blood cells and the subsequent cycles of replication and proliferation. In human populations, Plasmodium vivax is responsible for the most widely distributed recurring malaria infections whereas Plasmodium falciparum inflicts the most mortality and morbidity. One well-characterized family of adhesins involved in red blood cell invasion is the reticulocyte-binding-like protein homolog family, known as the RBL superfamily which includes the PfRh family in P. falciparum and PvRBP family in P. vivax. Here we report a collection of nanobodies against three members of this adhesin family, PfRh5, PfRh4 and PvRBP2b. Nanobodies against these Plasmodium adhesins bind with high affinity across several epitopes and can block receptor engagement and inhibit parasite invasion of red blood cells. Using computational design, we generated stabilized PfRh4 variants that encompass the conserved scaffold present in the PfRh and PvRBP families of adhesins and show that several variants with improved expression retained binding to mouse monoclonal antibodies, nanobodies and Complement Receptor 1, the human receptor for PfRh4. We also observed that most of the inhibitory nanobodies against the three antigens recognized the conserved structural scaffold that define this family of adhesins. These results demonstrate the potential of nanobodies to block malaria parasite invasion into red blood cells.

Sarcomeres, the basic repeating unit of striated muscle, are joined together by crosslinked actin filaments found at the boundaries of muscle sarcomeres, termed Z-discs. Z-discs play a key role in cardiac signalling and disease, however, the arrangement and function of many of the proteins present in the Z-disc remain to be understood. Here, we determined the organisation of 3 key proteins, ZASP, [a]-Actinin-2 and the Z1Z2 epitope of titin, located within the Z-disc. We fluorescently labelled these proteins in cardiac myofibrils using Adhirons specific to each protein and used interferometric photoactivated localization microscopy (iPALM) to obtain the 3D position of these proteins to a high precision (<10nm in x,y,z). We then used PERPL (Pattern Extraction from Relative Positions of Localisations) to analyse patterns in the relative positions of the proteins and reveal their underlying organisation. This analysis revealed that ZASP and [a]-Actinin-2 have a similar repeating organisation, but that the organisation of Z1Z2 is different.

Missense mutations in the TPM2 gene encoding skeletal muscle tropomyosin Tpm2.2 cause congenital myopathies associated with hyper- and hypocontractile phenotypes. Mutation-dependent defects in thin filament stability and length maintenance may contribute to sarcomere dysfunction. To address this possibility, four disease-associated substitutions in Tpm2.2 were analyzed: hypercontractile D20H and E181K, and hypocontractile E41K and N202K. Recombinant proteins were examined in vitro for their effects on actin filament polymerization, stability, and cofilin-2-dependent filament length regulation in the absence and presence of troponin (+Ca2+). Wild-type Tpm2.2 inhibited spontaneous actin polymerization and reduced polymerization cooperativity in the presence of cofilin-2. Hypercontractile substitutions D20H and E181K further decreased the polymerization rate, whereas hypocontractile variants had little effect. Under ATP-driven actomyosin interactions, E41K and N202K stabilized filaments, resulting in increased filament length, but this effect was abolished by troponin. All variants slightly decreased cofilin-2 affinity for F-actin without affecting cooperativity. Troponin prevented displacement of Tpm2.2 from the filament at increasing cofilin-2 occupancy, indicating concomitant binding of all proteins to the thin filament, consistent with a structural model based on high-resolution F-actin-Tpm-Tn and cofilactin structures.Tpm2.2-N202K inhibited cofilin-2-dependent depolymerization, whereas Tpm2.2-E181K increased susceptibility to depolymerization. Although cofilin-2 induced filament severing in all cases, the Tpm2.2-Tn complex protected filaments from disassembly. These findings support a model in which the Tpm2.2-Tn complex forms a cooperative regulatory strand that constrains filament dynamics and transmits structural perturbations along the filament. Disease-causing substitutions differentially alter filament length and stability, potentially contributing to the pathogenesis of myopathies.