A new anti-CRISPR gene promotes the spread of drug-resistance plasmids in Klebsiella pneumoniae.

The Klebsiella pneumoniae (K. pneumoniae, Kp) populations carrying both resistance-encoding and virulence-encoding mobile genetic elements (MGEs) significantly threaten global health. In this study, we identified a new anti-CRISPR gene (acrIE10) on a conjugative plasmid with self-target sequence in K. pneumoniae with type I-E* CRISPR-Cas system. AcrIE10 interacts with the Cas7* subunit of K. pneumoniae I-E* CRISPR-Cas system. The crystal structure of the AcrIE10-KpCas7* complex suggests that AcrIE10 suppresses the I-E* CRISPR-Cas by binding directly to Cas7 to prevent its hexamerization, thereby preventing the surveillance complex assembly and crRNA loading. Bioinformatic and functional analyses revealed that AcrIE10 is functionally widespread across diverse species. Our study reports a novel anti-CRISPR and highlights its potential role in spreading resistance and virulence among pathogens.

Discovery of Novel SIRT1/2 Inhibitors with Effective Cytotoxicity against Human Leukemia Cells.

The sirtuin enzyme family members, SIRT1 and SIRT2, play both tumor-promoting and tumor-suppressing roles, depending on the context and experimental conditions. Compounds that inhibit either SIRT1 or SIRT2 show promising antitumor effects in several types of cancer models, both in vitro and in vivo. The simultaneous inhibition of SIRT1 and SIRT2 is helpful in treating cancer by completely blocking p53 deacetylation, leading to cell death. However, only a few SIRT1/2 dual inhibitors have been developed. Here, we report the discovery of a novel series of SIRT1/2 dual inhibitors via a rational drug design that involved virtual screening and a substructure search. Eleven of the derived compounds exhibited high inhibitory activities, with IC50 < 5 µM and high specificity for both SIRT1 and SIRT2. Compounds hsa55 and PS9 strongly induced apoptosis and showed antiproliferative effects against human leukemia cell lines, which could be due to their ability to increase of p53 and α-tubulin acetylation, as we observed in MOLM-13 cells. Therefore, the new scaffolds of these compounds and their efficacy in leukemia cell lines provide important clues for the further development of novel anti-leukemia drugs.

Dracorhodin perchlorate inhibits osteoclastogenesis through repressing RANKL-stimulated NFATc1 activity.

Osteolytic skeletal disorders are caused by an imbalance in the osteoclast and osteoblast function. Suppressing the differentiation and resorptive function of osteoclast is a key strategy for treating osteolytic diseases. Dracorhodin perchlorate (D.P), an active component from dragon blood resin, has been used for facilitating wound healing and anti-cancer treatments. In this study, we determined the effect of D.P on osteoclast differentiation and function. We have found that D.P inhibited RANKL-induced osteoclast formation and resorbed pits of hydroxyapatite-coated plate in a dose-dependent manner. D.P also disrupted the formation of intact actin-rich podosome structures in mature osteoclasts and inhibited osteoclast-specific gene and protein expressions. Further, D.P was able to suppress RANKL-activated JNK, NF-κB and Ca2+ signalling pathways and reduces the expression level of NFATc1 as well as the nucleus translocation of NFATc1. Overall, these results indicated a potential therapeutic effect of D.P on osteoclast-related conditions.

The Prodomain-bound Form of Bone Morphogenetic Protein 10 Is Biologically Active on Endothelial Cells.

BMP10 is highly expressed in the developing heart and plays essential roles in cardiogenesis. BMP10 deletion in mice results in embryonic lethality because of impaired cardiac development. In adults, BMP10 expression is restricted to the right atrium, though ventricular hypertrophy is accompanied by increased BMP10 expression in a rat hypertension model. However, reports of BMP10 activity in the circulation are inconclusive. In particular, it is not known whether in vivo secreted BMP10 is active or whether additional factors are required to achieve its bioactivity. It has been shown that high-affinity binding of the BMP10 prodomain to the mature ligand inhibits BMP10 signaling activity in C2C12 cells, and it was proposed that prodomain-bound BMP10 (pBMP10) complex is latent. In this study, we demonstrated that the BMP10 prodomain did not inhibit BMP10 signaling activity in multiple endothelial cells, and that recombinant human pBMP10 complex, expressed in mammalian cells and purified under native conditions, was fully active. In addition, both BMP10 in human plasma and BMP10 secreted from the mouse right atrium were fully active. Finally, we confirmed that active BMP10 secreted from mouse right atrium was in the prodomain-bound form. Our data suggest that circulating BMP10 in adults is fully active and that the reported vascular quiescence function of BMP10 in vivo is due to the direct activity of pBMP10 and does not require an additional activation step. Moreover, being an active ligand, recombinant pBMP10 may have therapeutic potential as an endothelial-selective BMP ligand, in conditions characterized by loss of BMP9/10 signaling.

Heparin Binds Lamprey Angiotensinogen and Promotes Thrombin Inhibition through a Template Mechanism.

Lamprey angiotensinogen (l-ANT) is a hormone carrier in the regulation of blood pressure, but it is also a heparin-dependent thrombin inhibitor in lamprey blood coagulation system. The detailed mechanisms on how angiotensin is carried by l-ANT and how heparin binds l-ANT and mediates thrombin inhibition are unclear. Here we have solved the crystal structure of cleaved l-ANT at 2.7 Šresolution and characterized its properties in heparin binding and protease inhibition. The structure reveals that l-ANT has a conserved serpin fold with a labile N-terminal angiotensin peptide and undergoes a typical stressed-to-relaxed conformational change when the reactive center loop is cleaved. Heparin binds l-ANT tightly with a dissociation constant of ∼10 nm involving ∼8 monosaccharides and ∼6 ionic interactions. The heparin binding site is located in an extensive positively charged surface area around helix D involving residues Lys-148, Lys-151, Arg-155, and Arg-380. Although l-ANT by itself is a poor thrombin inhibitor with a second order rate constant of 500 m-1 s-1, its interaction with thrombin is accelerated 90-fold by high molecular weight heparin following a bell-shaped dose-dependent curve. Short heparin chains of 6-20 monosaccharide units are insufficient to promote thrombin inhibition. Furthermore, an l-ANT mutant with the P1 Ile mutated to Arg inhibits thrombin nearly 1500-fold faster than the wild type, which is further accelerated by high molecular weight heparin. Taken together, these results suggest that heparin binds l-ANT at a conserved heparin binding site around helix D and promotes the interaction between l-ANT and thrombin through a template mechanism conserved in vertebrates.

Molecular Mechanism of Z α1-Antitrypsin Deficiency.

The Z mutation (E342K) of α1-antitrypsin (α1-AT), carried by 4% of Northern Europeans, predisposes to early onset of emphysema due to decreased functional α1-AT in the lung and to liver cirrhosis due to accumulation of polymers in hepatocytes. However, it remains unclear why the Z mutation causes intracellular polymerization of nascent Z α1-AT and why 15% of the expressed Z α1-AT is secreted into circulation as functional, but polymerogenic, monomers. Here, we solve the crystal structure of the Z-monomer and have engineered replacements to assess the conformational role of residue Glu-342 in α1-AT. The results reveal that Z α1-AT has a labile strand 5 of the central ß-sheet A (s5A) with a consequent equilibrium between a native inhibitory conformation, as in its crystal structure here, and an aberrant conformation with s5A only partially incorporated into the central ß-sheet. This aberrant conformation, induced by the loss of interactions from the Glu-342 side chain, explains why Z α1-AT is prone to polymerization and readily binds to a 6-mer peptide, and it supports that annealing of s5A into the central ß-sheet is a crucial step in the serpins' metastable conformational formation. The demonstration that the aberrant conformation can be rectified through stabilization of the labile s5A by binding of a small molecule opens a potential therapeutic approach for Z α1-AT deficiency.

A redox switch in angiotensinogen modulates angiotensin release.

Blood pressure is critically controlled by angiotensins, which are vasopressor peptides specifically released by the enzyme renin from the tail of angiotensinogen-a non-inhibitory member of the serpin family of protease inhibitors. Although angiotensinogen has long been regarded as a passive substrate, the crystal structures solved here to 2.1 Å resolution show that the angiotensin cleavage site is inaccessibly buried in its amino-terminal tail. The conformational rearrangement that makes this site accessible for proteolysis is revealed in our 4.4 Å structure of the complex of human angiotensinogen with renin. The co-ordinated changes involved are seen to be critically linked by a conserved but labile disulphide bridge. Here we show that the reduced unbridged form of angiotensinogen is present in the circulation in a near 40:60 ratio with the oxidized sulphydryl-bridged form, which preferentially interacts with receptor-bound renin. We propose that this redox-responsive transition of angiotensinogen to a form that will more effectively release angiotensin at a cellular level contributes to the modulation of blood pressure. Specifically, we demonstrate the oxidative switch of angiotensinogen to its more active sulphydryl-bridged form in the maternal circulation in pre-eclampsia-the hypertensive crisis of pregnancy that threatens the health and survival of both mother and child.

Regulation of bone morphogenetic protein 9 (BMP9) by redox-dependent proteolysis.

BMP9, a member of the TGFß superfamily, is a homodimer that forms a signaling complex with two type I and two type II receptors. Signaling through high-affinity activin receptor-like kinase 1 (ALK1) in endothelial cells, circulating BMP9 acts as a vascular quiescence factor, maintaining endothelial homeostasis. BMP9 is also the most potent BMP for inducing osteogenic signaling in mesenchymal stem cells in vitro and promoting bone formation in vivo. This activity requires ALK1, the lower affinity type I receptor ALK2, and higher concentrations of BMP9. In adults, BMP9 is constitutively expressed in hepatocytes and secreted into the circulation. Optimum concentrations of BMP9 are essential to maintain the highly specific endothelial-protective function. Factors regulating BMP9 stability and activity remain unknown. Here, we showed by chromatography and a 1.9 Å crystal structure that stable BMP9 dimers could form either with (D-form) or without (M-form) an intermolecular disulfide bond. Although both forms of BMP9 were capable of binding to the prodomain and ALK1, the M-form demonstrated less sustained induction of Smad1/5/8 phosphorylation. The two forms could be converted into each other by changing the redox potential, and this redox switch caused a major alteration in BMP9 stability. The M-form displayed greater susceptibility to redox-dependent cleavage by proteases present in serum. This study provides a mechanism for the regulation of circulating BMP9 concentrations and may provide new rationales for approaches to modify BMP9 levels for therapeutic purposes.

Allosteric modulation of hormone release from thyroxine and corticosteroid-binding globulins.

The release of hormones from thyroxine-binding globulin (TBG) and corticosteroid-binding globulin (CBG) is regulated by movement of the reactive center loop in and out of the ß-sheet A of the molecule. To investigate how these changes are transmitted to the hormone-binding site, we developed a sensitive assay using a synthesized thyroxine fluorophore and solved the crystal structures of reactive loop cleaved TBG together with its complexes with thyroxine, the thyroxine fluorophores, furosemide, and mefenamic acid. Cleavage of the reactive loop results in its complete insertion into the ß-sheet A and a substantial but incomplete decrease in binding affinity in both TBG and CBG. We show here that the direct interaction between residue Thr(342) of the reactive loop and Tyr(241) of the hormone binding site contributes to thyroxine binding and release following reactive loop insertion. However, a much larger effect occurs allosterically due to stretching of the connecting loop to the top of the D helix (hD), as confirmed in TBG with shortening of the loop by three residues, making it insensitive to the S-to-R transition. The transmission of the changes in the hD loop to the binding pocket is seen to involve coherent movements in the s2/3B loop linked to the hD loop by Lys(243), which is, in turn, linked to the s4/5B loop, flanking the thyroxine-binding site, by Arg(378). Overall, the coordinated movements of the reactive loop, hD, and the hormone binding site allow the allosteric regulation of hormone release, as with the modulation demonstrated here in response to changes in temperature.

Crystal structure of protein Z-dependent inhibitor complex shows how protein Z functions as a cofactor in the membrane inhibition of factor X.

Protein Z (PZ) binds to PZ-dependent inhibitor (ZPI) and accelerates the inhibition of the coagulation protease, activated factor X (FXa), in the presence of phospholipids and Ca2+. A 2.3A resolution crystal structure of PZ complexed with ZPI shows that ZPI is a typical serine protease inhibitor and that PZ has a serine protease fold with distorted oxyanion hole and S1 pocket. The 2 molecules bind with fully complementary surfaces spanning over 2400A(2) and involving extensive ionic and hydrophobic interactions. ZPI has an unusual shutter region with a negatively charged residue buried within the hydrophobic core of the molecule. This unique Asp(213) is critical in maintaining the balanced metastability required for optimal protease inhibition, especially when PZ is bound, with its replacement with Asn resulting in increased thermal stability, but decreased efficiency of protease inhibition. The structure of ZPI shows negatively and positively charged surfaces on top of the molecule, in keeping with mutagenesis studies in this work indicating exosite interactions with FXa when it docks on top of ZPI. As modeled in this study, the gamma-carboxy-glutamic acid-containing domains of PZ and FXa enable them to bind to the same phospholipid surfaces on platelet and other membranes, with optimal proximity for the inhibition of FXa by the complexed ZPI.

Effect of eleutheroside B1 on non­coding RNAs and protein profiles of influenza A virus­infected A549 cells.

Influenza viruses often pose a serious threat to animals and human health. In an attempt to explore the potential of herbal medicine as a treatment for influenza virus infection, eleutheroside B1, a coumarin compound extracted from herba sarcandrae, was identified, which exhibited antiviral and anti­inflammatory activities against influenza A virus. In this study, high­throughput RNA sequencing and isobaric tags for relative and absolute quantification (iTRAQ) assays were performed to determine alterations in the non­coding RNA (ncRNA) transcriptome and proteomics. Bioinformatics and target prediction analyses were used to decipher the potential roles of altered ncRNAs in the function of eleutheroside B1. Furthermore, long ncRNA (lncRNA) and mRNA co­expressing networks were constructed to analyze the biological functions by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses. The analysis of RNA sequencing data revealed that 5 differentially expressed ncRNAs were upregulated and 3 ncRNAs were downregulated in the A549 cells infected with A/PR8/34/H1N1, with or without eleutheroside B1 treatment (PR8+eleu and PR8, respectively). Nuclear paraspeckle assembly transcript 1 (NEAT1) was differentially expressed between the PR8 and A549 cell groups. GO and KEGG pathway analyses indicated that eleutheroside B1 took advantage of the host cell biological processes and molecular function for its antiviral and anti­inflammatory activities, as well as for regulating cytokine­cytokine receptor interaction in the immune system, consistent with previous findings. The results of the iTRAQ assays indicated that L antigen family member 3 (LAGE3) protein, essential for tRNA processing, tRNA metabolic processes and ncRNA processing, was downregulated in the PR8+eleu compared with the PR8 group. In the present study, these comprehensive, large­scale data analysis enhanced the understanding of multiple aspects of the transcriptome and proteomics that are involved in the antiviral and anti­inflammatory activities of eleutheroside B1. These findings demonstrate the potential of eleutheroside B1 for use in the prevention and treatment of influenza A virus­mediated infections.

Heparin Blocks the Inhibition of Tissue Kallikrein 1 by Kallistatin through Electrostatic Repulsion.

Kallistatin, also known as SERPINA4, has been implicated in the regulation of blood pressure and angiogenesis, due to its specific inhibition of tissue kallikrein 1 (KLK1) and/or by its heparin binding ability. The binding of heparin on kallistatin has been shown to block the inhibition of KLK1 by kallistatin but the detailed molecular mechanism underlying this blockade is unclear. Here we solved the crystal structures of human kallistatin and its complex with heparin at 1.9 and 1.8 Å resolution, respectively. The structures show that kallistatin has a conserved serpin fold and undergoes typical stressed-to-relaxed conformational changes upon reactive loop cleavage. Structural analysis and mutagenesis studies show that the heparin binding site of kallistatin is located on a surface with positive electrostatic potential near a unique protruded 310 helix between helix H and strand 2 of ß-sheet C. Heparin binding on this site would prevent KLK1 from docking onto kallistatin due to the electrostatic repulsion between heparin and the negatively charged surface of KLK1, thus blocking the inhibition of KLK1 by kallistatin. Replacement of the acidic exosite 1 residues of KLK1 with basic amino acids as in thrombin resulted in accelerated inhibition. Taken together, these data indicate that heparin controls the specificity of kallistatin, such that kinin generation by KLK1 within the microcirculation will be locally protected by the binding of kallistatin to the heparin-like glycosaminoglycans of the endothelium.

Heterogeneous Klebsiella pneumoniae Co-infections Complicate Personalized Bacteriophage Therapy.

Multidrug-resistant (MDR) organisms have increased worldwide, posing a major challenge for the clinical management of infection. Bacteriophage is expected as potential effective therapeutic agents for difficult-to-treat infections. When performing bacteriophage therapy, the susceptibility of lytic bacteriophage to the target bacteria is selected by laboratory isolate from patients. The presence of a subpopulation in a main population of tested cells, coupled with the rapid development of phage-resistant populations, will make bacteriophage therapy ineffective. We aimed to treat a man with multifocal urinary tract infections of MDR Klebsiella pneumoniae by phage therapy. However, the presence of polyclonal co-infectious cells in his renal pelvis and bladder led to the failure of three consecutive phage therapies. After analysis, the patient was performed with percutaneous nephrostomy (PCN). A cocktail of bacteriophages was selected for activity against all 21 heterogeneous isolates and irrigated simultaneously via the kidney and bladder to eradicate multifocal colonization, combined with antibiotic treatment. Finally, the patient recovered with an obviously improved bladder. The success of this case provides valuable treatment ideas and solutions for phage treatment of complex infections. Clinical Trial Registration: www.chictr.org.cn, identifier ChiCTR1900020989.

Betulinic acid inhibits the migration and invasion of fibroblast-like synoviocytes from patients with rheumatoid arthritis.

The aggressive phenotype displayed by fibroblast-like synoviocytes (FLSs) contributes to cartilage and bone destruction in rheumatoid arthritis (RA). Betulinic acid has been demonstrated to have a positive therapeutic effect on tumor, inflammation and immune disorder, however, the effects of betulinic acid on RA FLSs have not been verified. Therefore, in the present study, we observed the effect of betulinic acid on the migration and invasion of RA FLSs and explored its underlying signal mechanisms. Our results showed that betulinic acid treatment suppressed the migration, invasion and reorganization of the actin cytoskeleton of RA FLSs. In addition, we found that the mRNA expression of IL-1ß, IL-6, IL-8 and IL-17A were markedly down-regulated by treatment with betulinic acid in TNF-α-induced RA FLSs. To gain insight into the molecular mechanisms, we evaluated the effect of betulinic acid on NF-κB activation in RA FLSs. The results indicated that betulinic acid treatment reduced the TNF-α-induced activation of NF-κB signal pathway and the NF-κB nuclear accumulation. We also observed that treatment with betulinic acid attenuated synovial inflammation and joint destruction in mice with CIA. Taken together, these results suggest that betulinic acid inhibits the migration and invasion of RA FLSs by blocking NF-κB signal pathway activation.

Identification of a natural inhibitor of methionine adenosyltransferase 2A regulating one-carbon metabolism in keratinocytes.

BACKGROUND: Psoriasis is a common chronic inflammatory skin disease which lacks effective strategies for the treatment. Natural compounds with biological activities are good tools to identify new targets with therapeutic potentials. Acetyl-11-keto-ß-boswellic acid (AKBA) is the most bioactive ingredient of boswellic acids, a group of compounds with anti-inflammatory and anti-cancer properties. Target identification of AKBA and metabolomics analysis of psoriasis helped to elucidate the molecular mechanism underlying its effect, and provide new target(s) to treat the disease. METHODS: To explore the targets and molecular mechanism of AKBA, we performed affinity purification, metabolomics analysis of HaCaT cells treated with AKBA, and epidermis of imiquimod (IMQ) induced mouse model of psoriasis and psoriasis patients. FINDINGS: AKBA directly interacts with methionine adenosyltransferase 2A (MAT2A), inhibited its enzyme activity, decreased level of S-adenosylmethionine (SAM) and SAM/SAH ratio, and reprogrammed one­carbon metabolism in HaCaT cells. Untargeted metabolomics of epidermis showed one­carbon metabolism was activated in psoriasis patients. Topical use of AKBA improved inflammatory phenotype of IMQ induced psoriasis-like mouse model. Molecular docking and site-directed mutagenesis revealed AKBA bound to an allosteric site at the interface of MAT2A dimer. INTERPRETATION: Our study extends the molecular mechanism of AKBA by revealing a new interacting protein MAT2A. And this leads us to find out the dysregulated one­carbon metabolism in psoriasis, which indicates the therapeutic potential of AKBA in psoriasis. FUND: The National Natural Science Foundation, the National Program on Key Basic Research Project, the Shanghai Municipal Commission, the Leading Academic Discipline Project of the Shanghai Municipal Education Commission.

Kaempferol inhibits the migration and invasion of rheumatoid arthritis fibroblast-like synoviocytes by blocking activation of the MAPK pathway.

In rheumatoid arthritis (RA), fibroblast-like synoviocytes (FLSs) play an essential role in cartilage destruction. Aggressive migration and invasion by FLSs significantly affect RA pathology. Kaempferol has been shown to inhibit cancer cell migration and invasion. However, the effects of kaempferol on RA FLSs have not been investigated. Our study aimed to determine the effects of kaempferol on RA both in vitro and in vivo. In vitro, cell migration and invasion were measured using scratch assays and the Boyden chamber method, respectively. The cytoskeletal reorganization of RA FLSs was evaluated by immunofluorescence staining. Matrix metalloproteinase (MMP) levels were measured by real-time PCR, and protein expression levels were measured by western blotting. In vivo, the effects of kaempferol were evaluated in mice with CIA. The results showed that kaempferol reduced migration, invasion and MMP expression in RA FLSs. In addition, we demonstrated that kaempferol inhibited reorganization of the actin cytoskeleton during cell migration. Moreover, kaempferol dramatically suppressed tumor necrosis factor (TNF)-α-induced MAPK activation without affecting the expression of TNF-α receptors. We also demonstrated that kaempferol attenuated the severity of arthritis in mice with CIA. Taken together, these results suggested that kaempferol inhibits the migration and invasion of FLSs in RA by blocking MAPK pathway activation without affecting the expression of TNF-α receptors.

Crystallization and crystallographic studies of kallistatin.

Kallistatin is a serine protease inhibitor (serpin) which specifically inhibits human tissue kallikrein; however, its inhibitory activity is inhibited by heparin. In order to elucidate the underlying mechanism, recombinant human kallistatin was prepared in Escherichia coli and the protein was crystallized by the sitting-drop vapour-diffusion method. X-ray diffraction data were collected to 1.9 Šresolution. The crystals were found to belong to space group P61, with unit-cell parameters a = 113.51, b = 113.51, c = 76.17 Å. Initial analysis indicated that the crystallized kallistatin was in a relaxed conformation, with its reactive-centre loop inserted in the central ß-sheet.

Small-volume chylous ascites after laparoscopic radical gastrectomy for gastric cancer: results from a large population-based sample.

AIM: To report the incidence and potential risk factors of small-volume chylous ascites (SVCA) following laparoscopic radical gastrectomy (LAG). METHODS: A total of 1366 consecutive gastric cancer patients who underwent LAG from January 2008 to June 2011 were enrolled in this study. We analyzed the patients based on the presence or absence of SVCA. RESULTS: SVCA was detected in 57 (4.17%) patients, as determined by the small-volume drainage (range, 30-100 mL/24 h) of triglyceride-rich fluid. Both univariate and multivariate analyses revealed that the total number of resected lymph nodes (LNs), No. 8 or No. 9 LN metastasis and N stage were independent risk factors for SVCA following LAG (P<0.05). Regarding hospital stay, there was a significant difference between the groups with and without SVCA (P<0.001). The 3-year disease-free and overall survival rates of the patients with SVCA were 47.4% and 56.1%, respectively, which were similar to those of the patients without SVCA (P>0.05). CONCLUSION: SVCA following LAG developed significantly more frequently in the patients with ≥32 harvested LNs, ≥3 metastatic LNs, or No. 8 or No. 9 LN metastasis. SVCA, which was successfully treated with conservative management, was associated with a prolonged hospital stay but was not associated with the prognosis.

Probing nanosecond motions of plasminogen activator inhibitor-1 by time-resolved fluorescence anisotropy.

Plasminogen activator inhibitor-1 (PAI-1) is a member of the serpin (serine protease inhibitor) superfamily. Like most serpins, the inhibitory function of PAI-1 relies on a flexible reactive centre loop (RCL) undertaking a striking conformational transition. We have investigated the conformational dynamics of the RCL of PAI-1 by time-resolved fluorescence anisotropy. A heterogeneous population model with three rotational correlation times has been employed to account for the "dip and rise" observed in fluorescence anisotropy decay curves. The RCL becomes almost fully solvent exposed and exhibits faster rotation when PAI-1 interacts with a RCL-mimicking octapeptide which blocks the loop insertion pathway, indicating that the RCL is well displaced from the protein surface; while the binding of Somatomedin B (SMB) domain of vitronectin, only induces small changes in the RCL. Comparison of the fluorescence lifetime and anisotropy decay of the wild-type PAI-1 with that of the stabilised mutant suggests that there would be no major structural differences between them. Our results indicate that in a native serpin, the P14 residue of the hinge region can flip in and out of the central beta-sheet A more readily than previously thought, which is likely an inherent property for serpins' protease inhibitory function.

The S-to-R transition of corticosteroid-binding globulin and the mechanism of hormone release.

Corticosteroids are transported in the blood by a serpin, corticosteroid-binding globulin (CBG), and their normally equilibrated release can be further triggered by the cleavage of the reactive loop of CBG. We report here the crystal structures of cleaved human CBG (cCBG) at 1.8-A resolution and its complex with cortisol at 2.3-A resolution. As expected, on cleavage, CBG undergoes the irreversible S-to-R serpin transition, with the cleaved reactive loops being fully incorporated into the central beta-sheet. A connecting loop of helix D, which is in a helix-like conformation in native CBG, unwinds and grossly perturbs the hormone binding site following beta-sheet expansion in the cCBG structure but shifts away from the binding site by more than 8 A following the binding of cortisol. Unexpectedly, on cortisol binding, the hormone binding site of cCBG adopts a configuration almost identical with that of the native conformer. We conclude that CBG has adapted an allosteric mechanism of the serpins to allow equilibrated release of the hormones by a flip-flop movement of the intact reactive loop into and out of the beta-sheet. The change in the hormone binding affinity results from a change in the flexibility or plasticity of the connecting loop, which modulates the configuration of the binding site.