Numerical simulation of ultrawideband supercontinuum generation covering a 2-20 µm waveband in cascaded all-soft-glass fiber.

We designed a cascaded all-soft-glass fiber structure and simulate midinfrared 2-20 µm ultrawideband supercontinuum (SC) generation numerically. The cascaded fiber structure consists of a 1.5 m I n F 3 fiber, a 0.2 m chalcogenide photonic crystal fiber, and a 0.2 m tellurium-based chalcogenide photonic crystal fiber. Using a 2 µm pulse pumping this cascaded structure, the generated SC covering the wavelengths longer than 20 µm has been demonstrated theoretically. The 30 dB bandwidth reaches 20.87 µm from 1.44 to 22.31 µm. The effect of different pulse widths on SC generation is considered. With the increase of peak power and the decrease of pulse width, the energy of SC in the 15-20 µm waveband increases gradually. The mechanism of SC broadening process has also been analyzed. The SC generation of more than 20 µm in this cascade structure is caused by the self-phase modulation, soliton effects, four-wave mixing, and redshifted dispersive wave. This method demonstrates the possibility of generating ultrawide bandwidth SCs up to a 20 µm waveband by a commercial 2 µm pump source and all-fiber structure.

Decaying dynamics of harmonic mode-locking in a SESAM-based mode-locked fiber laser.

The entire decaying dynamics of harmonic mode-locking (HML) are studied utilizing the dispersive Fourier transform (DFT) technique in a SESAM-based mode-locked fiber laser. It is unveiled that the harmonic solitons do not disappear directly, but undergo transitional processes from the higher-order HML to the lower-order HML and then to the fundamental mode-locking (FML), and finally vanish. The "big corner" can also exist in the decaying process rather than just in the buildup process of HML, and there is at least one "big corner" during the decaying process between the consecutive multi-pulsing states. The energy stabilization phase (ESP) cannot be observed during every transitional process. A breathing behavior and a vibrating soliton molecule are observed in the decaying process from the 2nd HML to the FML and in the decaying process of the FML, respectively. Our work would enrich the understanding of HML behaviors and may contribute to the laser designs.

High-efficiency mode-locked erbium-doped ZBLAN fiber laser around 2.8 µm by directly depositing Bi2S3 particles onto a cavity mirror.

Mid-infrared (MIR) pulsed lasers near a 3 µm waveband show great potential for the high absorption of water molecules and many important gas molecules. A passively Q-switched mode-locked (QSML) E r 3+-doped fluoride fiber laser with a low laser threshold and high slope efficiency around a 2.8 µm waveband is reported. The improvement is achieved by depositing bismuth sulfide (B i 2 S 3) particles onto the cavity mirror directly as a saturable absorber and using the cleaved end of the fluoride fiber as output directly. -QSML pulses begin to appear with the pump power of 280 mW. The repetition rate of the QSML pulses reaches a maximum of 33.59 kHz with the pump power of 540 mW. When the pump power is further increased, the output of the fiber laser switches from the QSML to the continuous-wave mode-locked operation with the repetition rate of 28.64 MHz and the slope efficiency of 12.2%. The results indicate that B i 2 S 3 is a promising modulator for the pulsed lasers near a 3 µm waveband, which paves the way for further development of various applications in MIR wavebands, including material processing, MIR frequency combs, and modern healthcare.

Bio-Inspired Hydrogel-Elastomer Actuator with Bidirectional Bending and Dynamic Structural Color.

In nature, some creatures can change their body shapes and surface colors simultaneously to respond to the external environments, which greatly inspired researchers in the development of color-tunable soft actuators. In this work, we present a facile method to prepare a smart hydrogel actuator that can bend bidirectionally and change color simultaneously, just like an octopus. The actuator is fabricated by elastomer/hydrogel bilayer and the hydrogel layer was decorated with thermoresponsive microgels as the photonic crystal blocks. Compared with the previously reported poly(N-isopropylacrylamide) hydrogel-based bilayer hydrogel actuators, which are generally limited to one-directional deformation, the elastomer/hydrogel bilayer actuator prepared in our work exhibits unique bidirectional bending behavior in accordance with the change of structural color. The bending degrees can be changed from -360° to 270° in response to solution temperatures ranging from 20 °C to 60 °C. At the same time, the surface color changes from red to green, and then to blue, covering the full visible light spectrum. The bending direction and degree of the hydrogel actuator can easily be adjusted by tuning the layer thickness ratio of the elastomer/hydrogel or the composition of the hydrogel. The color-tunable hydrogel-elastomer actuator reported in this work can achieve both programmable deformations and color-changing highly resembling the natural actuating behaviors of creatures.

Lactoferrin binding protein B - a bi-functional bacterial receptor protein.

Lactoferrin binding protein B (LbpB) is a bi-lobed outer membrane-bound lipoprotein that comprises part of the lactoferrin (Lf) receptor complex in Neisseria meningitidis and other Gram-negative pathogens. Recent studies have demonstrated that LbpB plays a role in protecting the bacteria from cationic antimicrobial peptides due to large regions rich in anionic residues in the C-terminal lobe. Relative to its homolog, transferrin-binding protein B (TbpB), there currently is little evidence for its role in iron acquisition and relatively little structural and biophysical information on its interaction with Lf. In this study, a combination of crosslinking and deuterium exchange coupled to mass spectrometry, information-driven computational docking, bio-layer interferometry, and site-directed mutagenesis was used to probe LbpB:hLf complexes. The formation of a 1:1 complex of iron-loaded Lf and LbpB involves an interaction between the Lf C-lobe and LbpB N-lobe, comparable to TbpB, consistent with a potential role in iron acquisition. The Lf N-lobe is also capable of binding to negatively charged regions of the LbpB C-lobe and possibly other sites such that a variety of higher order complexes are formed. Our results are consistent with LbpB serving dual roles focused primarily on iron acquisition when exposed to limited levels of iron-loaded Lf on the mucosal surface and effectively binding apo Lf when exposed to high levels at sites of inflammation.

miR-181d-5p-FOXP1 feedback loop modulates the progression of osteosarcoma.

MicroRNAs (miRNAs) are playing more and more fundamental roles in the progress of human cancers. miR-181d-5p has not been fully elucidated in human cancers. In this study, we aimed to investigate the biological function and mechanism of miR-181d-5p in osteosarcoma (OS). At first, the expression conditions of miR-181d-5p were identified in OS tissues and cell lines. Downregulation of miR-181d-5p was identified to be a significant prognostic factor for patients with OS. Using the bioinformatics analysis, the putative target mRNA (FOXP1) of miR-181d-5p was found out. Similarly, the expression conditions and prognostic value of FOXP1 were identified in OS. To validate the tumor suppressive role of miR-181d-5p in OS, gain or loss of function assays were carried out in OS cell lines. The results indicated the anti-oncogenic function of miR-181d-5p in OS. Subsequently, the oncogenic function of FOXP1 in OS was identified by functional assays. Rescue assays manifested that the oncogenic function of FOXP1 can be partially reversed by miR-181d-5p. Combining with the result of luciferase reporter assay, we confirmed that miR-181d-5p can act as a tumor suppressor in OS via targeting FOXP1. Further mechanism experiments revealed that FOXP1 can suppressed the transcription activity of miR-181d-5p by acting as a transcription inhibitor. In conclusion, our study revealed that miR-181d-5p-FOXP1 feedback loop modulates cell proliferation and metastasis in osteosarcoma.

Glycyrrhizin Suppresses RANKL-Induced Osteoclastogenesis and Oxidative Stress Through Inhibiting NF-κB and MAPK and Activating AMPK/Nrf2.

The treatment for osteoporosis involves inhibiting bone resorption and osteoclastogenesis. Glycyrrhizin (GLY) is a triterpenoid saponin glycoside known to be as the most medically efficacious component of the licorice plant. It has strong anti-inflammatory, antioxidant, and antitumor properties. We investigated the effect of GLY on osteoclastogenesis, bone resorption, and intracellular oxidative stress and its molecular mechanisms. In vitro osteoclastogenesis assays were performed using bone marrow monocytes with and without glycyrrhizin. We also evaluated the effects of glycyrrhizin on the secretion of TNF-α, IL-1ß, and IL-6 in LPS-stimulated RAW 264.7 cells using ELISA. The effects of glycyrrhizin on the expression of osteoclast-related genes, such as Nfatc1, c-fos, Trap, and cathepsin K (CK), were investigated by RT-PCR. Intracellular reactive oxygen species (ROS) were detected in receptor activator of nuclear factor kappa-Β ligand (RANKL)-stimulated osteoclasts in the presence and absence of glycyrrhizin. During the inhibition of osteoclastogenesis by glycyrrhizin, phosphorylation of AMPK, Nrf2, NF-κB, and MAPK was analyzed using western blotting. Our results showed that glycyrrhizin significantly inhibited RANKL-induced osteoclastogenesis, downregulated the expression of NFATc1, c-fos, TRAP, CK, DC-STAMP, and OSCAR, and inhibited p65, p38, and JNK. Glycyrrhizin was found to significantly decrease the secretion of inflammatory cytokines (TNF-α, IL-1ß, and IL-6). Additionally, glycyrrhizin reduced the formation of ROS in osteoclasts by inducing AMPK phosphorylation and nuclear transfer of NRF2, resulting in an upregulation of antioxidant enzymes, such as HO-1, NQO-1, and GCLC. In summary, we found that glycyrrhizin inhibited RANKL-induced osteoclastogenesis. It was also indicated that glycyrrhizin could reduce oxidative stress by inhibiting the MAPK and NF-κB pathways and activating the AMPK/NRF2 signaling. Therefore, glycyrrhizin may be used as an effective therapeutic agent against osteoporosis and bone resorption.

Transfer of obturator nerve for femoral nerve injury: an experiment study in rats.

BACKGROUND: Quadriceps palsy is mainly caused by proximal lesions in the femoral nerve. The obturator nerve has been previously used to repair the femoral nerve, although only a few reports have described the procedure, and the outcomes have varied. In the present study, we aimed to confirm the feasibility and effectiveness of this treatment in a rodent model using the randomized control method. METHODS: Sixty Sprague-Dawley rats were randomized into two groups: the experimental group, wherein rats underwent femoral neurectomy and obturator nerve transfer to the femoral nerve motor branch; and the control group, wherein rats underwent femoral neurectomy without nerve transfer. Functional outcomes were measured using the BBB score, muscle mass, and histological assessment. RESULTS: At 12 and 16 weeks postoperatively, the rats in the experimental group exhibited recovery to a stronger stretch force of the knee and higher BBB score, as compared to the control group (p < 0.05). The muscle mass and myofiber cross-sectional area of the quadriceps were heavier and larger than those in the control group (p < 0.05). A regenerated nerve with myelinated and unmyelinated fibers was observed in the experimental group. No significant differences were observed between groups at 8 weeks postoperatively (p > 0.05). CONCLUSIONS: Obturator nerve transfer for repairing femoral nerve injury was feasible and effective in a rat model, and can hence be considered as an option for the treatment of femoral nerve injury.

Insights into the bacterial transferrin receptor: the structure of transferrin-binding protein B from Actinobacillus pleuropneumoniae.

Pathogenic bacteria from the Neisseriaceae and Pasteurellacea families acquire iron directly from the host iron-binding glycoprotein, transferrin (Tf), in a process mediated by surface receptor proteins that directly bind host Tf, extract the iron, and transport it across the outer membrane. The bacterial Tf receptor is comprised of a surface exposed lipoprotein, Tf-binding protein B (TbpB), and an integral outer-membrane protein, Tf-binding protein A (TbpA), both of which are essential for survival in the host. In this study, we report the 1.98 A resolution structure of TbpB from the porcine pathogen Actinobacillus pleuropneumoniae, providing insights into the mechanism of Tf binding and the role of TbpB. A model for the complex of TbpB bound to Tf is proposed. Mutation of a single surface-exposed Phe residue on TbpB within the predicted interface completely abolishes binding to Tf, suggesting that the TbpB N lobe comprises the sole high-affinity binding region for Tf.

A method for measuring binding constants using unpurified in vivo biotinylated ligands.

Obtaining accurate kinetics and steady-state binding constants for biomolecular interactions normally requires pure and homogeneous protein preparations. Furthermore, in many cases, one of the ligands must be labeled. Over the past decade, several technologies have been introduced that allow for the measurement of kinetics constants for multiple different interactions in parallel. One such technology is bio-layer interferometry (BLI), which has been used to develop systems that can measure up to 96 biomolecular interactions simultaneously. However, despite the ever-increasing throughput of the tools available for measuring protein-protein interactions, the preparation of pure protein still remains a bottleneck in the process of producing high-quality kinetics data. Here, we show that high-quality binding data can be obtained using soluble lysate fractions containing protein that has been biotinylated in vivo using BirA and then applied to BLI sensors without further purification. Furthermore, we show that BirA ligase does not necessarily need to be co-overexpressed with the protein of interest for biotinylation of the biotin acceptor peptide to occur, suggesting that the activity of endogenous BirA in Escherichia coli is sufficient for producing enough biotinylated protein for a binding experiment.

Nonbinding site-directed mutants of transferrin binding protein B exhibit enhanced immunogenicity and protective capabilities.

Host-adapted Gram-negative bacterial pathogens from the Pasteurellaceae, Neisseriaceae, and Moraxellaceae families normally reside in the upper respiratory or genitourinary tracts of their hosts and rely on utilizing iron from host transferrin (Tf) for growth and survival. The surface receptor proteins that mediate this critical iron acquisition pathway have been proposed as ideal vaccine targets due to the critical role that they play in survival and disease pathogenesis in vivo. In particular, the surface lipoprotein component of the receptor, Tf binding protein B (TbpB), had received considerable attention as a potential antigen for vaccines in humans and food production animals but this has not translated into the series of successful vaccine products originally envisioned. Preliminary immunization experiments suggesting that host Tf could interfere with development of the immune response prompted us to directly address this question with site-directed mutant proteins defective in binding Tf. Site-directed mutants with dramatically reduced binding of porcine transferrin and nearly identical structure to the native proteins were prepared. A mutant Haemophilus parasuis TbpB was shown to induce an enhanced B-cell and T-cell response in pigs relative to native TbpB and provide superior protection from infection than the native TbpB or a commercial vaccine product. The results indicate that binding of host transferrin modulates the development of the immune response against TbpBs and that strategies designed to reduce or eliminate binding can be used to generate superior antigens for vaccines.

Doxorubicin-Loaded Micelles Based on Folic Acid Conjugated pH-Dependent Thermo-Sensitive Copolymer: In Vitro and In Vivo Evaluation.

In this paper, the doxorubicin (DOX)-loaded micelles were prepared based on a novel folic acid conjugated pH-dependent thermo-sensitive copolymer poly(D,L-lactic acid)-b-poly(N-isopropyl methacrylamide-co-N-isopropylmaelic acid-co-10-undecenoic acid) (PLA-PNNUA-FA) constructed to provide an active targeting drug delivery and triggered drug release system. The micelles were able to target tumors through the interaction between folic acid and its receptors which are overexpressed on the tumor cell membrane, and achieved pH-dependent thermo induced drug release in the intracellular mild acidic media such as endosomes and lysosomes after the micelles enter the cells. The results of cell assays and animal experiments showed that the micelles exhibited obvious tumor penetration efficiency in vivo, also improved DOX cell uptake and cytotoxicity in vitro. It was suggested that copolymer PLA-PNNUA-FA might be a potential targeted drug carrier to deliver chemotherapeutic drugs achieving better efficacy of chemotherapy.

Imaging findings of inflammatory myofibroblastic tumor of sigmoid colon: literature review and case report.

Inflammatory myofibroblastic tumor (IMT) is an intermediate tumor composed of differentiated myofibroblastic spindle cells with inflammatory cell infiltration. It can occur in all parts of the body, with the lungs being the most common, while the tissues outside the lungs, including the sigmoid colon, are rare. Herein, we present a case of a 10-year-old girl with sigmoid IMT who presented to our hospital with abdominal pain. An abdominal computed tomography (CT) revealed a well-defined, slightly low-density mass in her lower abdomen that was not clearly demarcated from the sigmoid colon. The mass showed significant uneven enhancement on contrast-enhanced CT and increased fluorine-18 fluorodeoxyglucose (18F-FDG) uptake on positron emission tomography (PET). Moreover, a systematic review of the published literature on sigmoid IMT was conducted and its clinical and radiographic features were summarized to increase the understanding of this rare disease.

Diagnostic Value of Hounsfield Units for Osteoporotic Thoracolumbar Vertebral Non-Compression Fractures in Elderly Patients with Low-Energy Injuries.

Background: Thoracolumbar vertebral fractures are common pathological fractures caused by osteoporosis in the elderly. These fractures are challenging to detect. This study aimed to evaluate the diagnostic value of Hounsfield units for osteoporotic thoracolumbar vertebral non-compression fractures in elderly patients with low-energy fractures. Methods: The retrospective case-control study included elderly patients diagnosed with osteoporotic thoracolumbar vertebral fractures and non-fractured patients who underwent computed tomography examinations for lumbar vertebra issues during July 2017 and June 2020. Results: This study included 216 patients with fractures (38 males and 178 females; average age: 77.28±8.68 years) and 124 patients without fractures (21 males and 103 females; average age: 75.35±9.57 years). The difference in Hounsfield units of the target (intermediate) vertebral body significantly differed between the two groups (54.74 ± 21.84 vs 5.86 ± 5.14; p<0.001). The ratios of Hounsfield units were also significantly different between the two groups (1.38 ± 1.60 vs 0.13 ± 0.23; p<0.001). The cut-off value for the difference in Hounsfield units to detect osteoporotic spine fractures was 25.35, with high sensitivity (98.5%), specificity (99.9%), and the area under the curve (AUC) (0.999, 95% CI: 0.999-1). The cut-off value for the odds ratio of Hounsfield units was 0.260, with high sensitivity (99.1%), specificity (92.7%), and AUC (0.970, 95% CI: 0.949-0.992). Conclusion: The difference between Hounsfield units and the odds ratio of Hounsfield units might help diagnose osteoporotic thoracolumbar vertebral non-compression fractures in elderly patients with low-energy fractures.

A secreted bacterial protein protects bacteria from cationic antimicrobial peptides by entrapment in phase-separated droplets.

Mammalian hosts combat bacterial infections through the production of defensive cationic antimicrobial peptides (CAPs). These immune factors are capable of directly killing bacterial invaders; however, many pathogens have evolved resistance evasion mechanisms such as cell surface modification, CAP sequestration, degradation, or efflux. We have discovered that several pathogenic and commensal proteobacteria, including the urgent human threat Neisseria gonorrhoeae, secrete a protein (lactoferrin-binding protein B, LbpB) that contains a low-complexity anionic domain capable of inhibiting the antimicrobial activity of host CAPs. This study focuses on a cattle pathogen, Moraxella bovis, that expresses the largest anionic domain of the LbpB homologs. We used an exhaustive biophysical approach employing circular dichroism, biolayer interferometry, cross-linking mass spectrometry, microscopy, size-exclusion chromatography with multi-angle light scattering coupled to small-angle X-ray scattering (SEC-MALS-SAXS), and NMR to understand the mechanisms of LbpB-mediated protection against CAPs. We found that the anionic domain of this LbpB displays an α-helical secondary structure but lacks a rigid tertiary fold. The addition of antimicrobial peptides derived from lactoferrin (i.e. lactoferricin) to the anionic domain of LbpB or full-length LbpB results in the formation of phase-separated droplets of LbpB together with the antimicrobial peptides. The droplets displayed a low rate of diffusion, suggesting that CAPs become trapped inside and are no longer able to kill bacteria. Our data suggest that pathogens, like M. bovis, leverage anionic intrinsically disordered domains for the broad recognition and neutralization of antimicrobials via the formation of biomolecular condensates.

TAX1BP1 and FIP200 orchestrate non-canonical autophagy of p62 aggregates for mouse neural stem cell maintenance.

Autophagy plays a pivotal role in diverse biological processes, including the maintenance and differentiation of neural stem cells (NSCs). Interestingly, while complete deletion of Fip200 severely impairs NSC maintenance and differentiation, inhibiting canonical autophagy via deletion of core genes, such as Atg5, Atg16l1, and Atg7, or blockade of canonical interactions between FIP200 and ATG13 (designated as FIP200-4A mutant or FIP200 KI) does not produce comparable detrimental effects. This highlights the likely critical involvement of the non-canonical functions of FIP200, the mechanisms of which have remained elusive. Here, utilizing genetic mouse models, we demonstrated that FIP200 mediates non-canonical autophagic degradation of p62/sequestome1, primarily via TAX1BP1 in NSCs. Conditional deletion of Tax1bp1 in fip200 hGFAP conditional knock-in (cKI) mice led to NSC deficiency, resembling the fip200 hGFAP conditional knockout (cKO) mouse phenotype. Notably, reintroducing wild-type TAX1BP1 not only restored the maintenance of NSCs derived from tax1bp1-knockout fip200 hGFAP cKI mice but also led to a marked reduction in p62 aggregate accumulation. Conversely, a TAX1BP1 mutant incapable of binding to FIP200 or NBR1/p62 failed to achieve this restoration. Furthermore, conditional deletion of Tax1bp1 in fip200 hGFAP cKO mice exacerbated NSC deficiency and p62 aggregate accumulation compared to fip200 hGFAP cKO mice. Collectively, these findings illustrate the essential role of the FIP200-TAX1BP1 axis in mediating the non-canonical autophagic degradation of p62 aggregates towards NSC maintenance and function, presenting novel therapeutic targets for neurodegenerative diseases.

Preparation and characterization of thermosensitive and folate functionalized Pluronic micelles.

In this study, thermosensitive and folate functionalized poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-ploy(N-isopropylacrylamide-co-hydroxyethyl methacrylate) (FA-Pluronic-PNH) copolymer was synthesized. The structure and molecular weight of the copolymer were confirmed by 1H NMR, FT-IR and GPC, respectively. The lower critical solution temperature (LCST) of the copolymer was 39.8 degrees C. By employing doxorubicin (DOX) as a model drug, folate receptor-targeted DOX-loaded micelles were further formed on the copolymer. The blank and DOX-loaded micelles both exhibited nearly spherical shapes and their average diameters were 35 nm and 50 nm, respectively. The in vitro release behaviors of the DOX-loaded micelles were temperature-dependent and the release rate of DOX at 42 degrees C (above LCST) was faster than that at 37 degrees C (below LCST). Furthermore, the cytotoxicity assays of free DOX and DOX-loaded micelles on human cervical cancer cell lines HeLa and human lung cancer cell lines A549 demonstrated that folate increased the cellular uptake of the micelles within targeted cells that vastly over-expressed folate receptors.

Steric and allosteric factors prevent simultaneous binding of transferrin-binding proteins A and B to transferrin.

The ability to acquire iron directly from host Tf (transferrin) is an adaptation common to important bacterial pathogens belonging to the Pasteurellaceae, Moraxellaceae and Neisseriaceae families. A surface receptor comprising an integral outer membrane protein, TbpA (Tf-binding protein A), and a surface-exposed lipoprotein, TbpB (Tf-binding protein B), mediates the iron acquisition process. TbpB is thought to extend from the cell surface for capture of Tf to initiate the process and deliver Tf to TbpA. TbpA functions as a gated channel for the passage of iron into the periplasm. In the present study we have mapped the effect of TbpA from Actinobacillus pleuropneumoniae on pTf (porcine Tf) using H/DX-MS (hydrogen/deuterium exchange coupled to MS) and compare it with a previously determined binding site for TbpB. The proposed TbpA footprint is adjacent to and potentially overlapping the TbpB-binding site, and induces a structural instability in the TbpB site. This suggests that simultaneous binding to pTf by both receptors would be hindered. We demonstrate that a recombinant TbpB lacking a portion of its anchor peptide is unable to form a stable ternary TbpA-pTf-TbpB complex. This truncated TbpB does not bind to a preformed Tf-TbpA complex, and TbpA removes pTf from a preformed Tf-TbpB complex. Thus the results of the present study support a model whereby TbpB 'hands-off' pTf to TbpA, which completes the iron removal and transport process.

Anchor peptide of transferrin-binding protein B is required for interaction with transferrin-binding protein A.

Gram-negative bacterial pathogens belonging to the Pasteurellaceae, Moraxellaceae, and Neisseriaceae families rely on an iron acquisition system that acquires iron directly from host transferrin (Tf). The process is mediated by a surface receptor composed of transferrin-binding proteins A and B (TbpA and TbpB). TbpA is an integral outer membrane protein that functions as a gated channel for the passage of iron into the periplasm. TbpB is a surface-exposed lipoprotein that facilitates the iron uptake process. In this study, we demonstrate that the region encompassing amino acids 7-40 of Actinobacillus pleuropneumoniae TbpB is required for forming a complex with TbpA and that the formation of the complex requires the presence of porcine Tf. These results are consistent with a model in which TbpB is responsible for the initial capture of iron-loaded Tf and subsequently interacts with TbpA through the anchor peptide. We propose that TonB binding to TbpA initiates the formation of the TbpB-TbpA complex and transfer of Tf to TbpA.

Conserved interaction between transferrin and transferrin-binding proteins from porcine pathogens.

Gram-negative porcine pathogens from the Pasteurellaceae family possess a surface receptor complex capable of acquiring iron from porcine transferrin (pTf). This receptor consists of transferrin-binding protein A (TbpA), a transmembrane iron transporter, and TbpB, a surface-exposed lipoprotein. Questions remain as to how the receptor complex engages pTf in such a way that iron is positioned for release, and whether divergent strains present distinct recognition sites on Tf. In this study, the TbpB-pTf interface was mapped using a combination of mass shift analysis and molecular docking simulations, localizing binding uniquely to the pTf C lobe for multiple divergent strains of Actinobacillus plueropneumoniae and suis. The interface was further characterized and validated with site-directed mutagenesis. Although targeting a common lobe, variants differ in preference for the two sublobes comprising the iron coordination site. Sublobes C1 and C2 participate in high affinity binding, but sublobe C1 contributes in a minor fashion to the overall affinity. Further, the TbpB-pTf complex does not release iron independent of other mediators, based on competitive iron binding studies. Together, our findings support a model whereby TbpB efficiently captures and presents iron-loaded pTf to other elements of the uptake pathway, even under low iron conditions.