Flexible Loop Research Articles

Computer-Aided Flexible Loops Engineering of Glutamate Dehydrogenase for Asymmetric Synthesis of Chiral Pesticides l-phosphinothricin.

The access to the enantiopure noncanonical amino acid l-phosphinothricin (l-PPT) by applying biocatalysts is highly appealing in organic chemistry. In this study, a NADH-dependent glutamate dehydrogenase from Lachnospiraceae bacterium (LbGluDH) was chosen for the asymmetric synthesis of l-PPT. Three flexible loops undergoing big conformational shifts during the catalysis were identified and rationally engineered following the initial mutagenesis. The enzyme's specific activity toward the key precursor of l-PPT, 2-oxo-4-[(hydroxy) (methyl) phosphinyl] butyric acid (PPO), was improved from negligible to 9 U/mg, and the Km value was reduced to 17 mM. The computational analysis showed that the modified loops broadened the enzyme's narrow tunnels, allowing the substrate to access the binding pocket and get closer to the crucial residue D165, thereby enhancing the catalytic process. Utilizing the variant as the catalyst, the preparation of l-PPT achieved a 100% conversion rate within 60 min, coupled with a stereoselectivity exceeding 99.9%, demonstrating its practical capacity for industrial application. Similar enhancement in catalytic activity was obtained applying the same strategy to a typical NADH-dependent GluDH from Pyrobaculum islandicum (PisGluDH), indicating the effectiveness of our strategy for the protein engineering of GluDHs targeted to the biosynthesis of unnatural compounds.

A Dynamic Loop in Halohydrin Dehalogenase HheG Regulates Activity and Enantioselectivity in Epoxide Ring Opening.

Halohydrin dehalogenase HheG and its homologues are remarkable enzymes for the efficient ring opening of sterically demanding internal epoxides using a variety of nucleophiles. The enantioselectivity of the respective wild-type enzymes, however, is usually insufficient for application and frequently requires improvement by protein engineering. We herein demonstrate that the highly flexible N-terminal loop of HheG, comprising residues 39 to 47, has a tremendous impact on the activity as well as enantioselectivity of this enzyme in the ring opening of structurally diverse epoxide substrates. Thus, highly active and enantioselective HheG variants could be accessed through targeted engineering of this loop. In this regard, variant M45F displayed almost 10-fold higher specific activity than wild type in the azidolysis of cyclohexene oxide, yielding the corresponding product (1S,2S)-2-azidocyclohexan-1-ol in 96%eeP (in comparison to 49%eeP for HheG wild type). Moreover, this variant was also improved regarding activity and enantioselectivity in the ring opening of cyclohexene oxide with other nucleophiles, demonstrating even inverted enantioselectivity with cyanide and cyanate. In contrast, a complete loop deletion yielded an inactive enzyme. Concomitant computational analyses of HheG M45F in comparison to wild type enzyme revealed that mutation M45F promotes the productive binding of cyclohexene oxide and azide in the active site by establishing noncovalent C-H ··π interactions between epoxide and F45. These interactions further position one of the two carbon atoms of the epoxide ring closer to the azide, resulting in higher enantioselectivity. Additionally, stable and enantioselective cross-linked enzyme crystals of HheG M45F were successfully generated after combination with mutation D114C. Overall, our study highlights that a highly flexible loop in HheG governs the enzyme's activity and selectivity in epoxide ring opening and should thus be considered in future protein engineering campaigns of HheG.

Sequence-Similar Protein Domain Pairs With Structural or Topological Dissimilarity.

For a variety of applications, protein structures are clustered by sequence similarity, and sequence-redundant structures are disregarded. Sequence-similar chains are likely to have similar structures, but significant structural variation, as measured with RMSD, has been documented for sequence-similar chains and found usually to have a functional explanation. Moving two neighboring stretches of backbone through each other may change the chain topology and alter possible folding paths. The size of this motion is compatible to a variation in a flexible loop. We search and find domains with alternate chain topology in CATH4.2 sequence families relatively independent of sequence identity and of structural similarity as measured by RMSD. Structural, topological, and functional representative sets should therefore keep sequence-similar domains not just with structural variation but also with topological variation. We present BCAlign that finds Alignment and superposition of protein Backbone Curves by optimizing a user chosen convex combination of structural derivation and derivation between the structure-based sequence alignment and an input sequence alignment. Steric and topological obstructions from deforming a curve into an aligned curve are then found by a previously developed algorithm. For highly sequence-similar domains, sequence-based structural alignment better represents the chains motion and generally reveals larger structural and topological variation than structure-based does. Fold-switching protein pairs have been reported to be most frequent between X-ray and NMR structures and estimated to be underrepresented in the PDB as the alternate configuration is harder to resolve. Here we similarly find chain topology most frequently altered between X-ray and NMR structures.

Coarse-Graining the Recognition of a Glycolipid by the C-Type Lectin Mincle Receptor.

Macrophage inducible Ca2+-dependent lectin (Mincle) receptor recognizes Mycobacterium tuberculosis glycolipids to trigger an immune response. This host membrane receptor is thus a key player in the modulation of the immune response to infection by M. tuberculosis and has emerged as a promising target for the development of new vaccines against tuberculosis. The recent development of the Martini 3 force field for coarse-grained (CG) molecular modeling allows the study of interactions of soluble proteins with small ligands which was not typically modeled well with the previous Martini 2 model. Here, we present a refined approach detailing a protocol for modeling interactions between a glycolipid and its receptor at a CG level using the Martini 3 force field. Using this approach, we studied Mincle and identified critical parameters governing ligand recognition, such as loop flexibility and the regulation of hydrophobic groove formation by calcium ions. In addition, we assessed ligand affinity using free energy perturbation calculations. Our results offer mechanistic insight into the interactions between Mincle and glycolipids, providing a basis for the rational design of molecules targeting this type of membrane receptors.

Resolving the Structure of a Guanine Quadruplex in TMPRSS2 Messenger RNA by Circular Dichroism and Molecular Modeling.

The presence of a guanine quadruplex in the opening reading frame of the messenger RNA coding for the transmembrane serine protease 2 (TMPRSS2) may pave the way to original anticancer and host-oriented antiviral strategy. Indeed, TMPRSS2 in addition to being overexpressed in different cancer types, is also related to the infection of respiratory viruses, including SARS-CoV-2, by promoting the cellular and viral membrane fusion through its proteolytic activity. The design of selective ligands targeting TMPRSS2 messenger RNA requires a detailed knowledge, at atomic level, of its structure. Therefore, we have used an original experimental-computational protocol to predict the first resolved structure of the parallel guanine quadruplex secondary structure in the RNA of TMPRSS2, which shows a rigid core flanked by a flexible loop. This represents the first atomic scale structure of the guanine quadruplex structure present in TMPRSS2 messenger RNA.

A NOVEL USE OF FINESSE FLEX LOOP FOR PROLIFERATIVE VITREORETINOPATHY (FINESSE LOOP- ASSISTED PHOTOCOAGULATION)

Purpose: To describe a novel use of the flexible nitinol loop membrane scraper (Finesse Flex Loop; Alcon, Forth Worth, TX) for applying an effective endolaser photocoagulation in proliferative vitreoretinopathy (PVR) cases. Methods: A 27- gauge nitinol flex loop was used to assist applying endolaser photocoagulation to enhance its effectiveness in a case with PVR. Results: A 66-year-old female was presented with PVR after an unsuccessful pars plana vitrectomy and silicone oil injection for rhegmatogenous retinal detachment in her right eye. The patient underwent 25- gauge pars plana vitrectomy, preretinal membrane removal, inferior retinectomy, the nitinol flex loop assisted endolaser photocoagulation, and silicone oil reinjection. After the silicone oil extraction, the retina was reattached and her visual acuity had improved from 0.1 (20/200) to 0.3 (20/60). Conclusion: The nitinol flex loop can be used to assist endolaser photocoagulation and enhance its effectiveness in PVR cases and reduce the retinal excision area which results unnecessary large retinal defects that may lead to redetachment of the retina.

Structural insights into the mechanism underlying the dual cofactor specificity of glyoxylate reductase from Acetobacter aceti in the β-hydroxyacid dehydrogenase family

The β-hydroxyacid dehydrogenase family exhibits diverse cofactor preferences: some enzymes favor NAD, others favor NADP, and a subset can utilize both NAD and NADPH. Glyoxylate reductase from Acetobacter aceti JCM 20276 (AacGR) exhibits a dual cofactor specificity for NADPH and NADH in its catalytic reduction of glyoxylate to glycolate. In contrast to conventional cofactor-discriminating motifs, NRX and DXX, found in NADP- and NAD-specific enzymes, respectively, AacGR has a TPS motif in the equivalent position. Here we report X-ray crystallographic analysis of AacGR in its ligand-free form, and in complexes with NADPH and NADH, revealing critical interactions: Ser41 of the TPS motif interacted with the 2′-phosphate group of NADPH, while no analogous interaction occurred with the ribose hydroxy groups of NADH. Moreover, the TPS motif resided within a characteristic β-turn-like structure adjacent to a long flexible loop. Site-directed mutagenesis and kinetic analyses suggest that Ser41 facilitates NADPH binding, while the lack of a direct interaction of the TPS motif with NADH may allow for NADH utilization. The conformational dynamics of the TPS-containing β-turn-like structure along with the flexible loop likely govern the dual cofactor specificity and catalytic turnover of AacGR.

Plumbagin, a novel TRPV2 inhibitor, ameliorates microglia activation and brain injury in a middle cerebral artery occlusion/reperfusion mouse model.

Transient receptor potential vanilloid 2 (TRPV2) is a Ca2+-permeable non-selective cation channel. Despite the significant roles of TRPV2 in immunological response, cancer progression and cardiac development, pharmacological probes of TRPV2 remain to be identified. We aimed to discover TRPV2 inhibitors and to elucidate their molecular mechanism of action. Fluorescence-based Ca2+ assay in HEK-293 cells expressing murine TRPV2 was used to identify plumbagin as a novel TRPV2 inhibitor. Patch-clamp, in silico docking and site-directed mutagenesis were applied to investigate the molecular mechanisms critical for plumbagin interaction. ELISA and qPCR were used to assess nitric oxide release and mRNA levels of inflammatory mediators, respectively. si-RNA interference was used to knock down TRPV2 expression, which was validated by western blotting. Neurological and histological analyses were used to examine brain injury of mice following middle cerebral artery occlusion/reperfusion (MCAO/R). Plumbagin is a potent TRPV2 negative allosteric modulator with an IC50 value of 0.85 μM, exhibiting >14-fold selectivity over TRPV1, TRPV3 and TRPV4. Plumbagin suppresses TRPV2 activity by decreasing the channel open probability without affecting the unitary conductance. Moreover, plumbagin binds to an extracellular pocket formed by the pore helix and flexible loop between transmembrane helices S5 and S6 of TRPV2. Plumbagin effectively suppresses LPS-induced inflammation of BV-2 microglia and ameliorates brain injury of MCAO/R mice. Plumbagin is a novel pharmacological probe to study TRPV2 pathophysiology. TRPV2 is a novel molecular target for the treatment of neuroinflammation and ischemic stroke.

Ligand-induced CaMKIIα hub Trp403 flip, hub domain stacking, and modulation of kinase activity.

γ-Hydroxybutyric acid (GHB) analogs are small molecules that bind competitively to a specific cavity in the oligomeric CaMKIIα hub domain. Binding affects conformation and stability of the hub domain, which may explain the neuroprotective action of some of these compounds. Here, we describe molecular details of interaction of the larger-type GHB analog 2-(6-(4-chlorophenyl)imidazo[1,2-b]pyridazine-2-yl)acetic acid (PIPA). Like smaller-type analogs, PIPA binding to the CaMKIIα hub domain promoted thermal stability. PIPA additionally modulated CaMKIIα activity under sub-maximal CaM concentrations and ultimately led to reduced substrate phosphorylation. A high-resolution X-ray crystal structure of a stabilized CaMKIIα (6x mutant) hub construct revealed details of the binding mode of PIPA, which involved outward placement of tryptophan 403 (Trp403), a central residue in a flexible loop close to the upper hub cavity. Small-angle X-ray scattering (SAXS) solution structures and mass photometry of the CaMKIIα wild-type hub domain in the presence of PIPA revealed a high degree of ordered self-association (stacks of CaMKIIα hub domains). This stacking neither occurred with the smaller compound 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA), nor when Trp403 was replaced with leucine (W403L). Additionally, CaMKIIα W403L hub was stabilized to a larger extent by PIPA compared to CaMKIIα hub wild type, indicating that loop flexibility is important for holoenzyme stability. Thus, we propose that ligand-induced outward placement of Trp403 by PIPA, which promotes an unforeseen mechanism of hub domain stacking, may be involved in the observed reduction in CaMKIIα kinase activity. Altogether, this sheds new light on allosteric regulation of CaMKIIα activity via the hub domain.

The influence of residue interaction on thermal stability of lipase based on dynamic graph embedding

Lipases with high thermal stability have greater application in industrial production. Focusing on wild-type lipase and their thermotolerant mutants, this study employs dynamic graph embedding to explore the change of spatiotemporal characteristics in residue-residue interactions, analyzing the relationship between residue mutations and the thermal stability of lipase. K-means clustering is used to analyze the synergistic relationships between residues, revealing that thermotolerant mutants achieve higher thermal stability by enhancing the synergistic interactions between secondary structures, resulting in a more balanced and tightly-knit internal structure. Critical residues are identified using anomaly node detection methods, showing that residues with spatiotemporal instability are mainly distributed on highly flexible loop regions. Specifically, mutations at Tyr89 and Asp132 restrict the motion range of the loops, making the lipase structure more stable. The study demonstrates that mutating highly flexible regions to increase structural rigidity can effectively enhance the thermal stability of lipases.

The potential role of concentrated solar power for off-grid green hydrogen and ammonia production

This paper investigates the potential role of concentrated solar power (CSP) in off-grid green electrolytic hydrogen and ammonia production using an open-source techno-economic assessment optimisation model. The green ammonia plant comprises a flexible Haber-Bosch loop, an electrolyser park, a desalination plant, storage systems (thermal, electrical, and hydrogen), and a power supply optionally including wind power, solar photovoltaic (PV), and CSP (solar tower or parabolic trough collectors). Various system configurations and cost assumptions are tested to evaluate when CSP technologies are economically viable for green fuel production. Results demonstrate that CSP and thermal energy storage (TES) technologies can play a minor but useful role in providing nighttime power supply to the ammonia plant, in combination with onshore wind power and electrical storage. Solar PV with 1-axis tracking remains the primary power provider, and the optimal system always includes large-scale hydrogen storage. In the cheapest configuration, the LCOA reaches 646 €/tNH3. With an 80% reduction in CSP and TES costs, CSP technology becomes a major player in power production. In scenarios where hydrogen storage is not feasible and the ammonia plant lacks full flexibility, a 20% cost reduction (to 2750 €/kW) is enough for them to become major power providers.

CONFORMATIONAL FLEXIBILITY IS A CRITICAL FACTOR IN DESIGNING BROAD-SPECTRUM HUMAN NOROVIRUS PROTEASE INHIBITORS.

Human norovirus (HuNoV) infection is a global health and economic burden. Currently, there are no licensed HuNoV vaccines or antiviral drugs available. The protease encoded by the HuNoV genome plays a critical role in virus replication by cleaving the polyprotein and is, therefore, an excellent target for developing small molecule inhibitors. While rupintrivir, a potent small-molecule inhibitor of several picornavirus proteases, effectively inhibits GI.1 protease, it is an order of magnitude less effective against GII protease. Other GI.1 protease inhibitors also tend to be less effective against GII proteases. To understand the structural basis for the potency difference, we determined the crystal structures of proteases of GI.1, pandemic GII.4 (Houston and Sydney), and GII.3 in complex with rupintrivir. These structures show that the open substrate pocket in GI protease binds rupintrivir without requiring significant conformational changes, whereas, in GII proteases, the closed pocket flexibly extends, reorienting arginine-112 in the BII-CII loop to accommodate rupintrivir. Structures of R112A protease mutants with rupintrivir, coupled with enzymatic and inhibition studies, suggest R112 is involved in displacing both substrate and ligands from the active site, implying a role in the release of cleaved products during polyprotein processing. Thus, the primary determinant for differential inhibitor potency between the GI and GII proteases is the increased flexibility in the BII-CII loop of the GII proteases caused by H-G mutation in this loop. Therefore, the inherent flexibility of the BII-CII loop in GII proteases is a critical factor to consider when developing broad-spectrum inhibitors for HuNoV proteases.

Molecular Mechanism of pH-Induced Protrusion Configuration Switching in Piscine Betanodavirus Implies a Novel Antiviral Strategy.

Many viruses contain surface spikes or protrusions that are essential for virus entry. These surface structures can thereby be targeted by antiviral drugs to treat viral infections. Nervous necrosis virus (NNV), a simple nonenveloped virus in the genus of betanodavirus, infects fish and damages aquaculture worldwide. NNV has 60 conspicuous surface protrusions, each comprising three protrusion domains (P-domain) of its capsid protein. NNV uses protrusions to bind to common receptors of sialic acids on the host cell surface to initiate its entry via the endocytic pathway. However, structural alterations of NNV in response to acidic conditions encountered during this pathway remain unknown, while detailed interactions of protrusions with receptors are unclear. Here, we used cryo-EM to discover that Grouper NNV protrusions undergo low-pH-induced compaction and resting. NMR and molecular dynamics (MD) simulations were employed to probe the atomic details. A solution structure of the P-domain at pH 7.0 revealed a long flexible loop (amino acids 311-330) and a pocket outlined by this loop. Molecular docking analysis showed that the N-terminal moiety of sialic acid inserted into this pocket to interact with conserved residues inside. MD simulations demonstrated that part of this loop converted to a β-strand under acidic conditions, allowing for P-domain trimerization and compaction. Additionally, a low-pH-favored conformation is attained for the linker connecting the P-domain to the NNV shell, conferring resting protrusions. Our findings uncover novel pH-dependent conformational switching mechanisms underlying NNV protrusion dynamics potentially utilized for facilitating NNV entry, providing new structural insights into complex NNV-host interactions with the identification of putative druggable hotspots on the protrusion.

Functional Linkers Support Targeting of Multivalent Tweezers to Taspase1.

Taspase 1 is a unique protease not only pivotal for embryonic development but also implicated in leukemias and solid tumors. As such, this enzyme is a promising while still challenging therapeutic target, and with its protein structure featuring a flexible loop preceding the active site a versatile model system for drug development. Supramolecular ligands provide a promising complementary approach to traditional small-molecule inhibitors. Recently, the multivalent arrangement of molecular tweezers allowed the successful targeting of Taspase 1's surface loop. With this study we now want to take the next logic step und utilize functional linker systems that not only allow the implementation of novel properties but also engage in protein surface binding. Consequently, we chose two different linker types differing from the original divalent assembly: a backbone with aggregation-induced emission (AIE) properties to enable monitoring of binding and a calix[4]arene scaffold initially pre-positioning the supramolecular binding units. With a series of four AIE-equipped ligands with stepwise increased valency we demonstrated that the functionalized AIE linkers approach ligand binding affinities in the nanomolar range and allow efficient proteolytic inhibition of Taspase 1. Moreover, implementation of the calix[4]arene backbone further enhanced the ligands' inhibitory potential, pointing to a specific linker contribution.

Structural and functional analysis of a bile salt hydrolase from the bison microbiome

The bile salt hydrolases (BSHs) are significant constituents of animal microbiomes. An evolving appreciation of their roles in health and disease has established them as targets of pharmacological inhibition. These bacterial enzymes belong to the N-terminal nucleophile superfamily and are best known to catalyze the deconjugation of glycine or taurine from bile salts to release bile acid substrates for transformation and or metabolism in the gastrointestinal tract. Here, we identify and describe the BSH from a common member of the Plains bison microbiome, Arthrobacter citreus (BSHAc). Steady-state kinetic analyses demonstrated that BSHAc is a broad-spectrum hydrolase with a preference for glycine-conjugates and deoxycholic acid (DCA). Second-order rate constants (kcat/KM) for BSHAc-catalyzed reactions of relevant bile salts-glyco- and tauro-conjugates of cholic acid and DCA- varied by ∼30-fold and measured between 1.4× 105 and 4.3× 106M-1s-1. Interestingly, a pan-BSH inhibitor named AAA-10 acted as a slow irreversible inhibitor of BSHAc with a rate of inactivation (kinact)of ∼2h-1 and a second order rate constant (kinact/KI) of ∼24M-1s-1 for the process. Structural characterization of BSHAc reacted with AAA-10 showed covalent modification of the N-terminal cysteine nucleophile, providing molecular details for an enzyme-stabilized product formed from this mechanism-based inhibitor's α-fluoromethyl ketone warhead. Structural comparison of the BSHs and BSH:inhibitor complexes highlighted the plasticity of the steroid-binding site, including a flexible loop that is variable across well-studied BSHs.

Diverse RNA Structures Induce PRC2 Dimerization and Inhibit Histone Methyltransferase Activity.

Methyltransferase PRC2 (Polycomb Repressive Complex 2) introduces histone H3K27 trimethylation, a repressive chromatin mark, to tune the differential expression of genes. PRC2 is precisely regulated by accessory proteins, histone post-translational modifications and, notably, RNA. Research on PRC2-associated RNA has mostly focused on the tight-binding G-quadruplex (G4) RNAs, which inhibit PRC2 enzymatic activity in vitro and in cells. Our recent cryo-EM structure provided a molecular mechanism for G4 RNA inactivating PRC2 via dimerization, but it remained unclear how diverse RNAs associate with and regulate PRC2. Here, we show that a single-stranded G-rich RNA and an atypical G4 structure called pUG-fold unexpectedly also mediate near-identical PRC2 dimerization resulting in inhibition of PRC2 methyltransferase activity. The conformational flexibility of arginine-rich loops within subunits EZH2 and AEBP2 of PRC2 can accommodate diverse RNA secondary structures, resulting in protein-RNA and protein-protein interfaces similar to those observed previously with G4 RNA. Furthermore, we address a recent report that failed to detect PRC2-associated RNAs in living cells by demonstrating the insensitivity of PRC2-RNA interaction to photochemical crosslinking. Our results support the significance of RNA-mediated PRC2 regulation by showing that this interaction is not limited to a single RNA secondary structure, consistent with the broad PRC2 transcriptome containing many G-tract RNAs incapable of folding into G4 structures.

The effect of the loop on the thermodynamic and kinetic of single base pair in pseudoknot.

RNA pseudoknots are RNA molecules with specialized three-dimensional structures that play important roles in various biological processes. To understand the functions and mechanisms of pseudoknots, it is essential to elucidate their structures and folding pathways. The most fundamental step in RNA folding is the opening and closing of a base pair. The effect of flexible loops on the base pair in pseudoknots remains unclear. In this work, we use molecular dynamics simulations and Markov state model to study the configurations, thermodynamic and kinetic of single base pair in pseudoknots. We find that the presence of the loop leads to a trap state. In addition, the rate-limiting step for the formation of base pair is the disruption of the trap state, rather than the open state to the closed state, which is quite different from the previous studies on non-pseudoknot RNA. For the thermodynamic parameters in pseudoknots, we find that the entropy difference upon opening the base pair between this simulation and the nearest-neighbor model results from the different entropy of different lengths of loop in solution. The thermodynamic parameters of the stack in pseudoknot are close to the nearest-neighbor parameters. The bases on the loop have different distribution patterns in different states, and the slow transition states of the loop are determined by the orientation of the bases.

Crystal structure of blue laccase BP76, a unique termite suicidal defense weapon

Aging workers of the termite Neocapritermes taracua can defend their colony by sacrificing themselves by body rupture, mixing the externally stored blue laccase BP76 with hydroquinones to produce a sticky liquid rich in toxic benzoquinones. Here, we describe the crystal structure of BP76 isolated from N.taracua in its native form. The structure reveals several stabilization strategies, including compact folding, glycosylation, and flexible loops with disulfide bridges and tight dimer interface. The remarkable stability of BP76 maintains its catalytic activity in solid state during the lifespan of N.taracua workers, providing old workers with an efficient defensive weapon to protect their colony.

Structure and replication of Pseudomonas aeruginosa phage JBD30.

Bacteriophages are the most abundant biological entities on Earth, but our understanding of many aspects of their lifecycles is still incomplete. Here, we have structurally analysed the infection cycle of the siphophage Casadabanvirus JBD30. Using its baseplate, JBD30 attaches to Pseudomonas aeruginosa via the bacterial type IV pilus, whose subsequent retraction brings the phage to the bacterial cell surface. Cryo-electron microscopy structures of the baseplate-pilus complex show that the tripod of baseplate receptor-binding proteins attaches to the outer bacterial membrane. The tripod and baseplate then open to release three copies of the tape-measure protein, an event that is followed by DNA ejection. JBD30 major capsid proteins assemble into procapsids, which expand by 7% in diameter upon filling with phage dsDNA. The DNA-filled heads are finally joined with 180-nm-long tails, which bend easily because flexible loops mediate contacts between the successive discs of major tail proteins. It is likely that the structural features and replication mechanisms described here are conserved among siphophages that utilize the type IV pili for initial cell attachment.

A pH-sensitive motif in an outer membrane protein activates bacterial membrane vesicle production

Outer membrane vesicles (OMVs) produced by Gram-negative bacteria have key roles in cell envelope homeostasis, secretion, interbacterial communication, and pathogenesis. The facultative intracellular pathogen Salmonella Typhimurium increases OMV production inside the acidic vacuoles of host cells by changing expression of its outer membrane proteins and modifying the composition of lipid A. However, the molecular mechanisms that translate pH changes into OMV production are not completely understood. Here, we show that the outer membrane protein PagC promotes OMV production through pH-dependent interactions between its extracellular loops and surrounding lipopolysaccharide (LPS). Structural comparisons and mutational studies indicate that a pH-responsive amino acid motif in PagC extracellular loops, containing PagC-specific histidine residues, is crucial for OMV formation. Molecular dynamics simulations suggest that protonation of histidine residues leads to changes in the structure and flexibility of PagC extracellular loops and their interactions with the surrounding LPS, altering membrane curvature. Consistent with that hypothesis, mimicking acidic pH by mutating those histidine residues to lysine increases OMV production. Thus, our findings reveal a mechanism for sensing and responding to environmental pH and for control of membrane dynamics by outer membrane proteins.