Pre-B cell receptor acts as a selectivity switch for galectin-1 at the pre-B cell surface.

Galectins are glycan-binding proteins translating the sugar-encoded information of cellular glycoconjugates into physiological activities, including immunity, cell migration, and signaling. Galectins also interact with non-glycosylated partners in the extracellular milieu, among which the pre-B cell receptor (pre-BCR) during B cell development. How these interactions might interplay with the glycan-decoding function of galectins is unknown. Here, we perform NMR experiments on native membranes to monitor Gal-1 binding to physiological cell surface ligands. We show that pre-BCR interaction changes Gal-1 binding to glycosylated pre-B cell surface receptors. At the molecular and cellular levels, we identify α2,3-sialylated motifs as key targeted epitopes. This targeting occurs through a selectivity switch increasing Gal-1 contacts with α2,3-sialylated poly-N-acetyllactosamine upon pre-BCR interaction. Importantly, we observe that this switch is involved in the regulation of pre-BCR activation. Altogether, this study demonstrates that interactions to non-glycosylated proteins regulate the glycan-decoding functions of galectins at the cell surface.

A molecular switch controls assembly of bacterial focal adhesions.

Cell motility universally relies on spatial regulation of focal adhesion complexes (FAs) connecting the substrate to cellular motors. In bacterial FAs, the Adventurous gliding motility machinery (Agl-Glt) assembles at the leading cell pole following a Mutual gliding-motility protein (MglA)-guanosine 5'-triphosphate (GTP) gradient along the cell axis. Here, we show that GltJ, a machinery membrane protein, contains cytosolic motifs binding MglA-GTP and AglZ and recruiting the MreB cytoskeleton to initiate movement toward the lagging cell pole. In addition, MglA-GTP binding triggers a conformational shift in an adjacent GltJ zinc-finger domain, facilitating MglB recruitment near the lagging pole. This prompts GTP hydrolysis by MglA, leading to complex disassembly. The GltJ switch thus serves as a sensor for the MglA-GTP gradient, controlling FA activity spatially.

The green cupredoxin CopI is a multicopper protein able to oxidize Cu(I).

Anthropogenic activities in agriculture and health use the antimicrobial properties of copper. This has led to copper accumulation in the environment and contributed to the emergence of copper resistant microorganisms. Understanding bacterial copper homeostasis diversity is therefore highly relevant since it could provide valuable targets for novel antimicrobial treatments. The periplasmic CopI protein is a monodomain cupredoxin comprising several copper binding sites and is directly involved in copper resistance in bacteria. However, its structure and mechanism of action are yet to be determined. To study the different binding sites for cupric and cuprous ions and to understand their possible interactions, we have used mutants of the putative copper binding modules of CopI and spectroscopic methods to characterize their properties. We show that CopI is able to bind a cuprous ion in its central histidine/methionine-rich region and oxidize it thanks to its cupredoxin center. The resulting cupric ion can bind to a third site at the N-terminus of the protein. Nuclear magnetic resonance spectroscopy revealed that the central histidine/methionine-rich region exhibits a dynamic behavior and interacts with the cupredoxin binding region. CopI is therefore likely to participate in copper resistance by detoxifying the cuprous ions from the periplasm.

Monitoring galactolipid digestion and simultaneous changes in lipid-bile salt micellar organization by real-time NMR spectroscopy.

The use of Nuclear Magnetic Resonance spectroscopy for studying lipid digestion in vitro most often consists of quantifying lipolysis products after they have been extracted from the reaction medium using organic solvents. However, the current sensitivity level of NMR spectrometers makes possible to avoid the extraction step and continuously quantify the lipids directly in the reaction medium. We used real-time 1H NMR spectroscopy and guinea pig pancreatic lipase-related protein 2 (GPLRP2) as biocatalyst to monitor in situ the lipolysis of monogalactosyl diacylglycerol (MGDG) in the form of mixed micelles with the bile salt sodium taurodeoxycholate (NaTDC). Residual substrate and lipolysis products (monogalactosyl monoacylglycerol (MGMG); monogalactosylglycerol (MGG) and octanoic acid (OA) were simultaneously quantified throughout the reaction thanks to specific proton resonances. Lipolysis was complete with the release of all MGDG fatty acids. These results were confirmed by thin layer chromatography (TLC) and densitometry after lipid extraction at different reaction times. Using diffusion-ordered NMR spectroscopy (DOSY), we could also estimate the diffusion coefficients of all the reaction compounds and deduce the hydrodynamic radius of the lipid aggregates in which they were present. It was shown that MGDG-NaTDC mixed micelles with an initial hydrodynamic radius rH of 7.3 ± 0.5 nm were changed into smaller micelles of NaTDC-MGDG-MGMG of 2.3 ± 0.5 nm in the course of the lipolysis reaction, and finally into NaTDC-OA mixed micelles (rH of 2.9 ± 0.5 nm) and water soluble MGG. These results provide a better understanding of the digestion of galactolipids by PLRP2, a process that leads to the complete micellar solubilisation of their fatty acids and renders their intestinal absorption possible.

Combating COVID-19 with charisma: Evidence on governor speeches in the United States.

Using field and laboratory data, we show that leader charisma can affect COVID-related mitigating behaviors. We coded a panel of U.S. governor speeches for charisma signaling using a deep neural network algorithm. The model explains variation in stay-at-home behavior of citizens based on their smart phone data movements, showing a robust effect of charisma signaling: stay-at-home behavior increased irrespective of state-level citizen political ideology or governor party allegiance. Republican governors with a particularly high charisma signaling score impacted the outcome more relative to Democratic governors in comparable conditions. Our results also suggest that one standard deviation higher charisma signaling in governor speeches could potentially have saved 5,350 lives during the study period (02/28/2020-05/14/2020). Next, in an incentivized laboratory experiment we found that politically conservative individuals are particularly prone to believe that their co-citizens will follow governor appeals to distance or stay at home when exposed to a speech that is high in charisma; these beliefs in turn drive their preference to engage in those behaviors. These results suggest that political leaders should consider additional "soft-power" levers like charisma-which can be learned-to complement policy interventions for pandemics or other public heath crises, especially with certain populations who may need a "nudge."

1H, 13C and 15N chemical shift assignments of the ZnR and GYF cytoplasmic domains of the GltJ protein from Myxococcus xanthus.

Bacterial cell motility is essential for a range of physiological phenomena such as nutrient sensing, predation, biofilm formation and pathogenesis. One of the most intriguing motilities is bacterial gliding, which is defined as the ability of some bacteria to move across surfaces without an external appendage. In Myxococcus xanthus, gliding motility depends on the assembly of focal adhesion complexes (FAC) which include the Glt mutiprotein complex and allow directional movement of individual cells (A-motility). Within the Glt multiprotein complex, GltJ is one of the key proteins involved in FAC assembly. In this work we report complete backbone and side chain 1H, 13C and 15N chemical shifts of the two cytoplasmic domains of GltJ, GltJ-ZnR (BMRB No. 51104) and GltJ-GYF (BMRB No. 51096). These data provide the first step toward the first high resolution structures of protein domains from the Glt machinery and the atomic level characterization of GltJ cytoplasmic activity during FAC assembly.

Flexibility of Oxidized and Reduced States of the Chloroplast Regulatory Protein CP12 in Isolation and in Cell Extracts.

In the chloroplast, Calvin-Benson-Bassham enzymes are active in the reducing environment created in the light by electrons from the photosystems. In the dark, these enzymes are inhibited, mainly caused by oxidation of key regulatory cysteine residues. CP12 is a small protein that plays a role in this regulation with four cysteine residues that undergo a redox transition. Using amide-proton exchange with solvent, measured by nuclear magnetic resonance (NMR) and mass-spectrometry, we confirmed that reduced CP12 is intrinsically disordered. Using real-time NMR, we showed that the oxidation of the two disulfide bridges is simultaneous. In oxidized CP12, the C23-C31 pair is in a region that undergoes a conformational exchange in the NMR-intermediate timescale. The C66-C75 pair is in the C-terminus that folds into a stable helical turn. We confirmed that these structural states exist in a physiologically relevant environment: a cell extract from Chlamydomonas reinhardtii. Consistent with these structural equilibria, the reduction is slower for the C66-C75 pair than for the C23-C31 pair. The redox mid-potentials for the two cysteine pairs differ and are similar to those found for glyceraldehyde 3-phosphate dehydrogenase and phosphoribulokinase, consistent with the regulatory role of CP12.

The unusual structure of Ruminococcin C1 antimicrobial peptide confers clinical properties.

The emergence of superbugs developing resistance to antibiotics and the resurgence of microbial infections have led scientists to start an antimicrobial arms race. In this context, we have previously identified an active RiPP, the Ruminococcin C1, naturally produced by Ruminococcus gnavus E1, a symbiont of the healthy human intestinal microbiota. This RiPP, subclassified as a sactipeptide, requires the host digestive system to become active against pathogenic Clostridia and multidrug-resistant strains. Here we report its unique compact structure on the basis of four intramolecular thioether bridges with reversed stereochemistry introduced posttranslationally by a specific radical-SAM sactisynthase. This structure confers to the Ruminococcin C1 important clinical properties including stability to digestive conditions and physicochemical treatments, a higher affinity for bacteria than simulated intestinal epithelium, a valuable activity at therapeutic doses on a range of clinical pathogens, mediated by energy resources disruption, and finally safety for human gut tissues.

Deciphering the specific interaction between the acyl carrier protein IacP and the T3SS-major hydrophobic translocator SipB from Salmonella.

Salmonella is a facultative intracellular pathogen that invades epithelial cells of the intestine using the SPI-1 Type 3 secretion System (T3SS). Insertion of the SPI-1 T3SS translocon is facilitated by acylation of the translocator SipB, which involves a protein-protein interaction with the acyl carrier protein IacP. Using nuclear magnetic resonance and biological tests, we identified the residues of IacP that are involved in the interaction with SipB. Our results suggest that the 4'-phosphopantetheine group that functionalizes IacP participates in the interaction. Its solvent exposition may rely on two residues highly conserved in acyl carrier proteins associated with T3SS. This study is the first to address the specificity of acyl carrier proteins associated with T3SS.

Ruminococcin C, a promising antibiotic produced by a human gut symbiont.

A major public health challenge today is the resurgence of microbial infections caused by multidrug-resistant strains. Consequently, novel antimicrobial molecules are actively sought for development. In this context, the human gut microbiome is an under-explored potential trove of valuable natural molecules, such as the ribosomally-synthesized and post-translationally modified peptides (RiPPs). The biological activity of the sactipeptide subclass of RiPPs remains under-characterized. Here, we characterize an antimicrobial sactipeptide, Ruminococcin C1, purified from the caecal contents of rats mono-associated with Ruminococcus gnavus E1, a human symbiont. Its heterologous expression and post-translational maturation involving a specific sactisynthase establish a thioether network, which creates a double-hairpin folding. This original structure confers activity against pathogenic Clostridia and multidrug-resistant strains but no toxicity towards eukaryotic cells. Therefore, the Ruminococcin C1 should be considered as a valuable candidate for drug development and its producer strain R. gnavus E1 as a relevant probiotic for gut health enhancement.

Structural and functional insights into the periplasmic detector domain of the GacS histidine kinase controlling biofilm formation in Pseudomonas aeruginosa.

Pseudomonas aeruginosa is an opportunistic pathogenic bacterium responsible for both acute and chronic infections and has developed resistance mechanisms due to its ability to promote biofilm formation and evade host adaptive immune responses. Here, we investigate the functional role of the periplasmic detector domain (GacSPD) from the membrane-bound GacS histidine kinase, which is one of the key players for biofilm formation and coordination of bacterial lifestyles. A gacS mutant devoid of the periplasmic detector domain is severely defective in biofilm formation. Functional assays indicate that this effect is accompanied by concomitant changes in the expression of the two RsmY/Z small RNAs that control activation of GacA-regulated genes. The solution NMR structure of GacSPD reveals a distinct PDC/PAS α/ß fold characterized by a three-stranded ß-sheet flanked by α-helices and an atypical major loop. Point mutations in a putative ligand binding pocket lined by positively-charged residues originating primarily from the major loop impaired biofilm formation. These results demonstrate the functional role of GacSPD, evidence critical residues involved in GacS/GacA signal transduction system that regulates biofilm formation, and document the evolutionary diversity of the PDC/PAS domain fold in bacteria.

NMR assignments of the GacS histidine-kinase periplasmic detection domain from Pseudomonas aeruginosa PAO1.

Pseudomonas aeruginosa is a highly adaptable opportunistic pathogen. It can infect vulnerable patients such as those with cystic fibrosis or hospitalized in intensive care units where it is responsible for both acute and chronic infection. The switch between these infections is controlled by a complex regulatory system involving the central GacS/GacA two-component system that activates the production of two small non-coding RNAs. GacS is a histidine kinase harboring one periplasmic detection domain, two inner-membrane helices and three H1/D1/H2 cytoplasmic domains. By detecting a yet unknown signal, the GacS histidine-kinase periplasmic detection domain (GacSp) is predicted to play a key role in activating the GacS/GacA pathway. Here, we present the chemical shift assignment of 96 % of backbone atoms (HN, N, C, Cα, Cß and Hα), 88 % aliphatic hydrogen atoms and 90 % of aliphatic carbon atoms of this domain. The NMR-chemical shift data, on the basis of Talos server secondary structure predictions, reveal that GacSp consists of 3 ß-strands, 3 α-helices and a major loop devoid of secondary structures.

(1)H, (15)N and (13)C resonance assignments of the C-terminal domain of Vibrio cholerae TolA protein.

Vibrio cholerae is the bacterial causative agent of the human disease cholera. Non-pathogenic bacterium can be converted to pathogenic following infection by a filamentous phage, CTXΦ, that carries the cholera toxin encoding genes. A crucial step during phage infection requires a direct interaction between the CTXΦ minor coat protein (pIII(CTX)) and the C-terminal domain of V. cholerae TolA protein (TolAIIIvc). In order to get a better understanding of TolA function during the infection process, we have initiated a study of the V. cholerae TolAIII domain by 2D and 3D heteronuclear NMR. With the exception of the His-tag (H123-H128), 97 % of backbone (1)H, (15)N and (13)C resonances were assigned and the side chain assignments for 92 % of the protein were obtained (BMRB deposit with accession number 25689).

Pre-B cell receptor binding to galectin-1 modifies galectin-1/carbohydrate affinity to modulate specific galectin-1/glycan lattice interactions.

Galectins are glycan-binding proteins involved in various biological processes including cell/cell interactions. During B-cell development, bone marrow stromal cells secreting galectin-1 (GAL1) constitute a specific niche for pre-BII cells. Besides binding glycans, GAL1 is also a pre-B cell receptor (pre-BCR) ligand that induces receptor clustering, the first checkpoint of B-cell differentiation. The GAL1/pre-BCR interaction is the first example of a GAL1/unglycosylated protein interaction in the extracellular compartment. Here we show that GAL1/pre-BCR interaction modifies GAL1/glycan affinity and particularly inhibits binding to LacNAc containing epitopes. GAL1/pre-BCR interaction induces local conformational changes in the GAL1 carbohydrate-binding site generating a reduction in GAL1/glycan affinity. This fine tuning of GAL1/glycan interactions may be a strategic mechanism for allowing pre-BCR clustering and pre-BII cells departure from their niche. Altogether, our data suggest a novel mechanism for a cell to modify the equilibrium of the GAL1/glycan lattice involving GAL1/unglycosylated protein interactions.

Identification of a Src kinase SH3 binding site in the C-terminal domain of the human ErbB2 receptor tyrosine kinase.

Overexpression of the ErbB2 receptor tyrosine kinase is associated with most aggressive tumors in breast cancer patients and is thus one of the main investigated therapeutic targets. Human ErbB2 C-terminal domain is an unstructured anchor that recruits specific adaptors for signaling cascades resulting in cell growth, differentiation and migration. Herein, we report the presence of a SH3 binding motif in the proline rich unfolded ErbB2 C-terminal region. NMR analysis of this motif supports a PPII helix conformation and the binding to Fyn-SH3 domain. The interaction of a kinase of the Src family with ErbB2 C-terminal domain could contribute to synergistic intracellular signaling and enhanced oncogenesis.

¹H, ¹³C and ¹5N backbone and side-chain chemical shift assignments for reduced unusual thioredoxin Patrx2 of Pseudomonas aeruginosa.

The gram-negative organism Pseudomonas aeruginosa is an opportunistic human pathogen and a leading cause of hospital-acquired infections. In P. aeruginosa PAO1, three cytoplasmic thioredoxins have been identified. An unusual thioredoxin (Patrx2) (108 amino acids) encoded by the PA2694 gene, is identified as a new thioredoxin-like protein based on sequence homology. Thioredoxin is a ubiquitous protein, which serves as a general protein disulfide oxidoreductase. Patrx2 present an atypical active site CGHC. We report the nearly complete (1)H, (13)C and (15)N resonance assignments of reduced Patrx2. 2D and 3D heteronuclear NMR experiments were performed with uniformly (15)N-, (13)C-labelled Patrx2, resulting in 97.2% backbone and 92.5% side-chain (1)H, (13)C and (15)N resonance assignments for the reduced form. (BMRB deposits with accession number 18130).

Structural basis for galectin-1-dependent pre-B cell receptor (pre-BCR) activation.

During B cell differentiation in the bone marrow, the expression and activation of the pre-B cell receptor (pre-BCR) constitute crucial checkpoints for B cell development. Both constitutive and ligand-dependent pre-BCR activation modes have been described. The pre-BCR constitutes an immunoglobulin heavy chain (Igµ) and a surrogate light chain composed of the invariant λ5 and VpreB proteins. We previously showed that galectin-1 (GAL1), produced by bone marrow stromal cells, is a pre-BCR ligand that induces receptor clustering, leading to efficient pre-BII cell proliferation and differentiation. GAL1 interacts with the pre-BCR via the unique region of λ5 (λ5-UR). Here, we investigated the solution structure of a minimal λ5-UR motif that interacts with GAL1. This motif adopts a stable helical conformation that docks onto a GAL1 hydrophobic surface adjacent to its carbohydrate binding site. We identified key hydrophobic residues from the λ5-UR as crucial for the interaction with GAL1 and for pre-BCR clustering. These residues involved in GAL1-induced pre-BCR activation are different from those essential for autonomous receptor activation. Overall, our results indicate that constitutive and ligand-induced pre-BCR activation could occur in a complementary manner.

Structural and mechanistic insights into unusual thiol disulfide oxidoreductase.

Cytoplasmic desulfothioredoxin (Dtrx) from the anaerobe Desulfovibrio vulgaris Hildenborough has been identified as a new member of the thiol disulfide oxidoreductase family. The active site of Dtrx contains a particular consensus sequence, CPHC, never seen in the cytoplasmic thioredoxins and generally found in periplasmic oxidases. Unlike canonical thioredoxins (Trx), Dtrx does not present any disulfide reductase activity, but it presents instead an unusual disulfide isomerase activity. We have used NMR spectroscopy to gain insights into the structure and the catalytic mechanism of this unusual Dtrx. The redox potential of Dtrx (-181 mV) is significantly less reducing than that of canonical Trx. A pH dependence study allowed the determination of the pK(a) of all protonable residues, including the cysteine and histidine residues. Thus, the pK(a) values for the thiol group of Cys(31) and Cys(34) are 4.8 and 11.3, respectively. The His(33) pK(a) value, experimentally determined for the first time, differs notably as a function of the redox states, 7.2 for the reduced state and 4.6 for the oxidized state. These data suggest an important role for His(33) in the molecular mechanism of Dtrx catalysis that is confirmed by the properties of mutant DtrxH33G protein. The NMR structure of Dtrx shows a different charge repartition compared with canonical Trx. The results presented are likely indicative of the involvement of this protein in the catalysis of substrates specific of the anaerobe cytoplasm of DvH. The study of Dtrx is an important step toward revealing the molecular details of the thiol-disulfide oxidoreductase catalytic mechanism.

MEMO associated with an ErbB2 receptor phosphopeptide reveals a new phosphotyrosine motif.

Tyrosine phosphorylations are essential in signal transduction. Recently, a new type of phosphotyrosine binding protein, MEMO (Mediator of ErbB2-driven cell motility), has been reported to bind specifically to an ErbB2-derived phosphorylated peptide encompassing Tyr-1227 (PYD). Structural and functional analyses of variants of this peptide revealed the minimum sequence required for MEMO recognition. Using a docking approach we have generated a structural model for MEMO/PYD complex and compare this new phosphotyrosine motif to SH2 and PTB phosphotyrosine motives.

1H, 13C and 15N chemical shift assignments of the thioredoxin from the obligate anaerobe Desulfovibrio vulgaris Hildenborough.

Thioredoxins are ubiquitous key antioxidant enzymes which play an essential role in cell defense against oxidative stress. They maintain the redox homeostasis owing to the regulation of thiol-disulfide exchange. In the present paper, we report the full resonance assignments of (1)H, (13)C and (15)N atoms for the reduced and oxidized forms of Desulfovibrio vulgaris Hildenborough thioredoxin 1 (Trx1). 2D and 3D heteronuclear NMR experiments were performed using uniformly (15)N-, (13)C-labelled Trx1. Chemical shifts of 97% of the backbone and 90% of the side chain atoms were obtained for the oxidized and reduced form (BMRB deposits with accession number 17299 and 17300, respectively).