Measurement of the ^{18}O(α, γ)^{22}Ne Reaction Rate at JUNA and Its Impact on Probing the Origin of SiC Grains

The ^{18}O(α,γ)^{22}Ne reaction is critical for AGB star nucleosynthesis due to its connection to the abundances of several key isotopes, such as ^{21}Ne and ^{22}Ne. However, the ambiguous resonance energy and spin-parity of the dominant 470 keV resonance leads to substantial uncertainty in the ^{18}O(α,γ)^{22}Ne reaction rate for the temperature of interest. We have measured the resonance energies and strengths of the low-energy resonances in ^{18}O(α,γ)^{22}Ne at the Jinping Underground Nuclear Astrophysics experimental facility (JUNA) with improved precision. The key 470 keV resonance energy has been measured to be E_{α}=474.0±1.1  keV, with such high precision achieved for the first time. The spin-parity of this resonance state is determined to be 1^{-}, removing discrepancies in the resonance strengths in earlier studies. The results significantly improve the precision of the ^{18}O(α,γ)^{22}Ne reaction rates by up to about 10 times compared with the previous data at typical AGB temperatures of 0.1-0.3 GK. We demonstrate that such improvement leads to precise ^{21}Ne abundance predictions, with an impact on probing the origin of meteoritic stardust SiC grains from AGB stars.

[Analysis of the characteristics of delayed high-degree atrioventricular block after transcatheter aortic valve replacement]

Objective: To investigate the characteristics of delayed high-degree atrioventricular block (DHAVB) after transcatheter aortic valve replacement (TAVR). Methods: One hundred and seventy-six patients who underwent TAVR with a self-extending valve between May 2014 and November 2018 in the Department of Cardiology, West China Hospital of Sichuan University, were retrospectively enrolled, including 101 males and 75 females, aged 54-92 (73±7) years, and the data were collected during the perioperative and 30 d follow-up periods. According to the occurrence of HAVB after TAVR, 160 patients were divided into no-HAVB group (145 cases) and DHAVB group (15 cases), except 16 patients who developed HAVB within 2 days after TAVR. Baseline data, intraoperative data, and immediate postoperative ECG characteristics were compared between the two groups, and logistic regression models were used to analyze the factors associated with the occurrence of DHAVB after TAVR. Meanwhile, the diagnostic ability of the postoperative routine 12-lead ECG for DHAVB was evaluated using the ambulatory ECG findings as the standard diagnosis. Results: The incidence of DHAVB was 8.5% (15/176) and occurred at 5 (4, 6) d. Compared with the no-HAVB group. The percentage of no new conduction block on the immediate postoperative ECG was lower in the DHAVB group [6/15 vs 66.2%(96/145), P=0.044], and the percentage of new right bundle branch block on the immediate postoperative ECG was higher [4/15 vs 3.4%(5/145), P=0.002]. Multifactorial logistic regression analysis showed that right bundle branch block on the immediate postoperative ECG [OR (95%CI):6.60 (1.26-34.47), P=0.025] was an associated factor for the development of DHAVB after TAVR. The specificity of postoperative routine 12-lead ECG for the diagnosis of DHAVB was 100% (145/145), but the sensitivity was only 73.3% (11/15). Conclusions: The incidence of DHAVB after TAVR is also high in Chinese. The immediate postoperative ECG characteristics of patients who underwent TAVR are associated with DHAVB events, and applying these characteristics to risk stratify patients may optimize the management of DHAVB after TAVR.

[Chinese expert consensus on management of dyslipidemia in the elderly]

Dyslipidemia is an important risk factor of atherosclerotic cardiovascular disease (ASCVD). Statins delay the occurrence and development of ASCVD, and reduce the risk of cardiovascular events and death. Due to safety concerns, there exist insufficient use of lipid-lowering agents and a high withdrawal rate of the agents in the elderly. To promote the prevention and treatment of ASCVD, this expert consensus is issued and focuses on the management of dyslipidemia of Chinese elderly basing on the clinical evidence of the use of lipid-lowering drugs by the elderly, and the lipid management guidelines and expert consensus recommendations at home and abroad.

Deep Underground Laboratory Measurement of ^{13}C(α,n)^{16}O in the Gamow Windows of the s and i Processes

The ^{13}C(α,n)^{16}O reaction is the main neutron source for the slow-neutron-capture process in asymptotic giant branch stars and for the intermediate process. Direct measurements at astrophysical energies in above-ground laboratories are hindered by the extremely small cross sections and vast cosmic-ray-induced background. We performed the first consistent direct measurement in the range of E_{c.m.}=0.24 to 1.9 MeV using the accelerators at the China Jinping Underground Laboratory and Sichuan University. Our measurement covers almost the entire intermediate process Gamow window in which the large uncertainty of the previous experiments has been reduced from 60% down to 15%, eliminates the large systematic uncertainty in the extrapolation arising from the inconsistency of existing datasets, and provides a more reliable reaction rate for the studies of the slow-neutron-capture and intermediate processes along with the first direct determination of the alpha strength for the near-threshold state.

Abnormal keratin expression pattern in prurigo nodularis epidermis

Background

Prurigo nodularis (PN) is a highly pruritic, chronic dermatosis and difficult to treat. PN lesions are characterized by existence of many hyperkeratotic, erosive papules and nodules. However, the pathogenesis of PN still remains unelucidated.

Aim

To clarify the keratin role in the epidermis hyperproliferation, the keratin expression pattern in the PN lesional skin.

Methods

In this study, we enrolled 24 patients with PN and 9 healthy control volunteers. K1/K10, K5/K14, K6/K16/K17 expression pattern were investigated by using immunohistochemical staining.

Results

The lesional skin consists of the thickened spinous layers, in which active cell division was found. K5/K14 were upregulated in PN lesional epidermis, the staining signal localized in the basal layer and lower suprabasal layers. Hyperproliferation‐associated K6 was found in all layers of epidermal lesional skin, especially in the spinous layers. In contrast, K16 was only detected in the basal and lower suprabasal layers, K17 was observed in the basal and spinous layers. Terminal differential keratins K1/K10 were upregulated, detected in the pan‐epidermis, but spared in the basal and low suprabasal layers.

Conclusion

The keratinocytes enter an alternative differentiation pathway, which are responsible for the activated keratinocyte phenotype, abnormal keratins expression potentially contributes to the keratinocytes proliferation, subsequently lead to increased lesional skin epidermis thickness, hyperkeratiosis and alteration of skin barrier properties.

[Preliminary clinical experience of the novel transcatheter aortic valve system Prizvalve® for the treatment of severe aortic stenosis]

Objective: To evaluate the safety and efficacy of transcatheter aortic valve implantation (TAVI) with the novel Prizvalve^® system in treating severe aortic stenosis. Methods: This is a single-center, prospective, single-arm, observational study. A total of 11 patients with severe aortic stenosis with high risk or inappropriate for conventional surgical aortic valve replacement (SAVR) were included, and TAVI was achieved with the Prizvalve^® system between March 2021 and May 2021 in West China Hospital. Transthoracic echocardiography (TTE) was performed immediately after prosthesis implantation to evaluate mean transaortic gradient and maximal transaortic velocity. The device success rate was calculated, which was defined as (1) the device being delivered via the access, deployed, implanted and withdrawn, (2) mean transaortic gradient<20 mmHg (1 mmHg=0.133 kPa) or a maximal transaortic velocity<3 m/s post TAVI, and without severe aortic regurgitation or paravalvular leak post TAVI. TTE was performed at 30 days after the surgery, and all-cause mortality as well as the major cardiovascular adverse events (including acute myocardial infarction, disabling hemorrhagic or ischemic stroke) up to 30 days post TAVI were analyzed. Results: The age of 11 included patients were (78.1±6.3) years, with 8 males. A total of 10 patients were with NYHA functional class Ⅲ or Ⅳ. Devices were delivered via the access, deployed, implanted and withdrawn successfully in all patients. Post-implant mean transaortic gradient was (7.55±4.08) mmHg and maximal transaortic velocity was (1.78±0.44) m/s, and both decreased significantly as compared to baseline levels (both P<0.05). No severe aortic regurgitation or paravalvular leak was observed post TAVI. Device success was achieved in all the 11 patients. No patient died or experienced major cardiovascular adverse events up to 30 days post TAVI. Mean transaortic gradient was (9.45±5.07) mmHg and maximal transaortic velocity was (2.05±0.42) m/s at 30 days post TAVI, which were similar as the values measured immediately post TAVI (both P>0.05). Conclusions: TAVI with the Prizvalve^® system is a feasible and relatively safe procedure for patients with severe aortic stenosis and at high risk or inappropriate for SAVR. Further clinical studies could be launched to obtain more clinical experience with Prizvalve^® system.

Direct Measurement of the Astrophysical ^{19}F(p,αγ)^{16}O Reaction in the Deepest Operational Underground Laboratory

Fluorine is one of the most interesting elements in nuclear astrophysics, where the ^{19}F(p,α)^{16}O reaction is of crucial importance for Galactic ^{19}F abundances and CNO cycle loss in first generation Population III stars. As a day-one campaign at the Jinping Underground Nuclear Astrophysics experimental facility, we report direct measurements of the essential ^{19}F(p,αγ)^{16}O reaction channel. The γ-ray yields were measured over E_{c.m.}=72.4-344  keV, covering the Gamow window; our energy of 72.4 keV is unprecedentedly low, reported here for the first time. The experiment was performed under the extremely low cosmic-ray-induced background environment of the China JinPing Underground Laboratory, one of the deepest underground laboratories in the world. The present low-energy S factors deviate significantly from previous theoretical predictions, and the uncertainties are significantly reduced. The thermonuclear ^{19}F(p,αγ)^{16}O reaction rate has been determined directly at the relevant astrophysical energies.

Plant MIR167e-5p Inhibits Enterocyte Proliferation by Targeting β-Catenin

MicroRNAs (miRNAs) are important negative regulators of genes involved in physiological and pathological processes in plants and animals. It is worth exploring whether plant miRNAs play a cross-kingdom regulatory role in animals. Herein, we found that plant MIR167e-5p regulates the proliferation of enterocytes in vitro. A porcine jejunum epithelial cell line (IPEC-J2) and a human colon carcinoma cell line (Caco-2) were treated with 0, 10, 20, and 40 pmol of synthetic 2′-O-methylated plant MIR167e-5p, followed by a treatment with 20 pmol of MIR167e-5p for 0, 24, 48, and 72 h. The cells were counted, and IPEC-J2 cell viability was determined by the MTT and EdU assays at different time points. The results showed that MIR167e-5p significantly inhibited the proliferation of enterocytes in a dose- and time-dependent manner. Bioinformatics prediction and a luciferase reporter assay indicated that MIR167e-5p targets β-catenin. In IPEC-J2 and Caco-2 cells, MIR167e-5p suppressed proliferation by downregulating β-catenin mRNA and protein levels. MIR167e-5p relieved this inhibition. Similar results were achieved for the β-catenin downstream target gene c-Myc and the proliferation-associated gene PCNA. This research demonstrates that plant MIR167e-5p can inhibit enterocyte proliferation by targeting the β-catenin pathway. More importantly, plant miRNAs may be a new class of bioactive molecules for epigenetic regulation in humans and animals.

Plant MIR156 regulates intestinal growth in mammals by targeting the Wnt/β-catenin pathway

MicroRNAs (miRNAs) are important negative regulators of genes involved in physiological and pathological processes in plants and animals. Recent studies have shown that miRNAs might regulate gene expression among different species in a cross-kingdom manner. However, the specific roles of plant miRNAs in animals remain poorly understood and somewhat. Herein, we found that plant MIR156 regulates proliferation of intestinal cells both in vitro and in vivo. Continuous administration of a high plant miRNA diet or synthetic MIR156 elevated MIR156 levels and inhibited the Wnt/β-catenin signaling pathway in mouse intestine. Bioinformatics predictions and luciferase reporter assays indicated that MIR156 targets Wnt10b. In vitro, MIR156 suppressed proliferation by downregulating the Wnt10b protein and upregulating β-catenin phosphorylation in the porcine jejunum epithelial (IPEC-J2) cell line. Lithium chloride and an MIR156 inhibitor relieved this inhibition. This research is the first to demonstrate that plant MIR156 inhibits intestinal cell proliferation by targeting Wnt10b. More importantly, plant miRNAs may represent a new class of bioactive molecules that act as epigenetic regulators in humans and other animals.

Exploration of long noncoding RNA in bovine milk exosomes and their stability during digestion in vitro

Previous studies have demonstrated that bovine milk contains mRNA and microRNA that are largely encapsulated in milk-derived exosomes. However, little information is available about long noncoding RNAs (lncRNA) in bovine milk. Increasing evidence suggests that lncRNA are of particular interest given their key role in gene expression and development. We performed a comprehensive analysis of lncRNA in bovine milk exosomes by RNA sequencing. We used a validated human in vitro digestion model to investigate the stability of lncRNA encapsulated in bovine milk exosomes during the digestion process. We identified 3,475 novel lncRNA and 6 annotated lncRNA. The lncRNA shared characteristics with those of other mammals in terms of length, exon number, and open reading frames. However, lncRNA showed higher expression than mRNAs. We selected 12 lncRNA of high-expression abundance and identified them by PCR. Gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses showed that lncRNA regulate immune function, osteoblastogenesis, neurodevelopment, reproduction, cell proliferation, and cell-cell communication. We also investigated the 12 lncRNA using quantitative real-time PCR to reveal their expression profiles in milk exosomes during different stages of lactation (colostrum 2 d, 30 d, 150 d, and 270 d); their resulting expression levels in milk exosomes showed variations across the stages. A digestion experiment showed that bovine milk exosome lncRNA was resistant to in vitro digestion with different digestive juices, including saliva, gastric juice, pancreatic juice, and bile juice. Taken together, these results show for the first time that cow milk contains lncRNA, and that their abundance varied at different stages of lactation. As expected, bovine milk exosomal lncRNA were stable during in vitro digestion. These findings provide a basis for further understanding of the physiological role of milk lncRNA.

[Association between food intake and the serum total cholesterol level among adults in 9 regions of China]

Objective: To analyze the association between food intake and the serum total cholesterol(TC) level in adult Chinese population. Methods: This study included apparent healthy residents aged between 20 and 70 years who participated in the eighth round of China Health and Nutrition Study (CHNS) in 9 regions of China including Liaoning, Helongjiang, Shandong, Jiangsu, Henan, Hunan, Hubei, Guizhou provinces and Guangxi Zhuang Autonomous Region in 2009. The association between the serum TC level and the diet and other related factors were analyzed. Results: (1) Linear trend test showed that age, body mass index, waist circumference, systolic and diastolic blood pressure were positively related with serum TC level in 20-45 and 46-70 years old male and female (all P<0.01), while physical activity level was negatively related with serum TC level in 20-45 and 46-70 years old male and 46-70 years old female (all P<0.01). (2) Multivariate linear regression analysis showed that livestock meat intake was positively related with serum TC level in following populations: 20-45 years old male, β=0.133, P<0.001; 46-70 years old male, β=0.102, P=0.001; 20-45 years old male, β=0.065, P=0.041; years old female, β=0.059, P=0.045. Vegetable oil intake was negatively related with serum TC level in 20-45 years old male (β=-0.071, P=0.021). Saturated fat intake was positively related with serum TC level in 46-70 years old female (β=0.084, P=0.019). Total dietary cholesterol intake was positively related with serum TC level in male (20-45 years old, β=0.067, P=0.021; 46-70 years old, β=0.070, P=0.012), but not in female (20-45 years old, β=0.007, P=0.809; 46-70 years old, β=0.038, P=0.144). Cholesterol intake from livestock meat was positively related with serum TC level in male (20-45 years old, β=0.156, P<0.001; 46-70 years old, β=0.099, P=0.001), and positively related with serum TC level in 46-70 years old female (β=0.063, P=0.028). Cholesterol intake from egg was positively related with serum TC level in 46-70 years old male (β=0.056, P=0.032). Conclusions: Age, body mass index, waist circumference, blood pressure, blood pressure, and physical activity level are related with serum TC level. Cholesterol intake from livestock meat was the main food source that related with serum TC level, and the serum TC increases at a relatively small level after intake of total dietary cholesterol.

Novel NPHS1 splice site mutations in a Chinese child with congenital nephrotic syndrome

Congenital nephrotic syndrome (CNS) is defined as heavy proteinuria or nephrotic syndrome occurring before 3 months of age. It is characterized by early onset and progresses to end-stage renal disease. Recently, several genes associated with CNS have been identified, including NPHS1 and NPHS2. Mutations in the NPHS1 gene have been identified in patients with CNS in Finland with relatively high frequency. Thus far, only a few case reports about CNS have described an NPHS1 mutation in China. In this study, mutational analyses of NPHS1 and NPHS2 were performed in a Chinese child with CNS. Mutations were analyzed in all exons and exon/intron boundaries of NPHS1 and NPHS2 in the patient and his parents as well as in 50 unrelated controls using polymerase chain reaction and direct sequencing techniques. No mutations were detected in NPHS2. A novel splice site mutation (IVS11+1G>A) within intron 11 and a missense mutation within exon 8 (c.928G>A) in the NPHS1 gene were detected in the child. The child's mother had normal urinalysis and a c.928G>A (D310N) heterozygous mutation, and his father had normal urinalysis and IVS11+1G>A. These were not identified in the 50 unrelated controls. The novel splice site mutation of IVS11+1G>A and a missense mutation at c.928G>A in NPHS1 were found to cause CNS in this Chinese child.

Identification and bioinformatic analysis of a putative calcium-dependent protein kinase (CDPK6) from Toxoplasma gondii

Toxoplasma gondii is recognized as an opportunistic human pathogen with a worldwide distribution. Development of effective vaccines is considered the only ideal way to control T. gondii infection. However, only one live vaccine is commercially available for use in sheep and goats. Therefore, the identification of more effective antigenic proteins is very important. In this study, we identified a novel putative calcium-dependent protein kinase of T. gondii, TgCDPK6, and further analyzed its potential antigenicity using a bioinformatic approach. The physical and chemical characteristics, transmembrane domain, epitopes, advanced structure, and functional sites of TgCDPK6 were predicted by multiple bioinformatic approaches. Twenty-six post-translational modification sites were identified in the protein. The secondary structure showed that 58.35% amino acids of TgCDPK6 are exposed to the solvent interface, and the high hydrophilic domains were distributed in amino acid positions 21-59, 68-81, 156-205, 245-271, 280-294, 297-324, 334-356, 367-393, 474-498, and 543-553. The advanced structure of TgCDPK6 was developed by a homology modeling method and was validated by PROCHECK, which showed that most amino acid residues were in the most favored regions. Using these analyses, 10 potential epitopes were predicted. The results indicated that TgCDPK6 could be a vaccine candidate antigen against T. gondii.

Missing posterior teeth and risk of temporomandibular disorders

There is disagreement about the association between missing posterior teeth and the presence of temporomandibular disorders (TMD). Here, the purpose was to investigate whether the number of missing posterior teeth, their distribution, age, and gender are associated with TMD. Seven hundred and forty-one individuals, aged 21-60 years, with missing posterior teeth, 386 with and 355 without TMD, were included. Four variables-gender, age, the number of missing posterior teeth, and the number of dental quadrants with missing posterior teeth-were analyzed with a logistic regression model. All four variables-gender (OR = 1.59, men = 1, women = 2), age (OR = 0.98), the number of missing posterior teeth (OR = 0.51), and the number of dental quadrants with missing posterior teeth (OR = 7.71)-were entered into the logistic model (P < 0.01). The results indicate that individuals who lose posterior teeth, with fewer missing posterior teeth but in more quadrants, have a higher prevalence of TMD, especially young women.

A novel HLA-B*15 allele, B*9534, identified by sequence-based typing in the Chinese population

We report here the identification of a novel human leukocyte antigen-B*9534 allele that was detected by polymerase chain reaction sequence-based typing.

Uptake of HIV-1 tat protein mediated by low-density lipoprotein receptor-related protein disrupts the neuronal metabolic balance of the receptor ligands

Neurological disorders develop in most people infected with human immunodeficiency virus type 1 (HIV-1). However, the underlying mechanisms remain largely unknown. Here we report that binding of HIV-1 transactivator (Tat) protein to low-density lipoprotein receptor-related protein (LRP) promoted efficient uptake of Tat into neurons. LRP-mediated uptake of Tat was followed by translocation to the neuronal nucleus. Furthermore, the binding of Tat to LRP resulted in substantial inhibition of neuronal binding, uptake and degradation of physiological ligands for LRP, including alpha2-macroglobulin, apolipoprotein E4, amyloid precursor protein and amyloid beta-protein. In a model of macaques infected with a chimeric strain of simian-human immunodeficiency virus, increased staining of amyloid precursor protein was associated with Tat expression in the brains of simian-human immunodeficiency virus-infected macaques with encephalitis. These results indicate that HIV-1 Tat may mediate HIV-1-induced neuropathology through a pathway involving disruption of the metabolic balance of LRP ligands and direct activation of neuronal genes.

Dominant role of local dipoles in stabilizing uncompensated charges on a sulfate sequestered in a periplasmic active transport protein

Electrostatic interactions are among the key factors determining the structure and function of proteins. Here we report experimental results that illuminate the functional importance of local dipoles to these interactions. The refined 1.7-A X-ray structure of the liganded form of the sulfate-binding protein, a primary sulfate active transport receptor of Salmonella typhimurium, shows that the sulfate dianion is completely buried and bound by hydrogen bonds (mostly main-chain peptide NH groups) and van der Waals forces. The sulfate is also closely linked, via one of these peptide units, to a His residue. It is also adjacent to the N-termini of three alpha-helices, of which the two shortest have their C-termini "capped" by Arg residues. Site-directed mutagenesis of the recombinant Escherichia coli sulfate receptor had no effect on sulfate-binding activity when an Asn residue was substituted for the positively charged His and the two Arg (changed singly and together) residues. These results, combined with other observations, further solidify the idea that stabilization of uncompensated charges in a protein is a highly localized process that involves a collection of local dipoles, including those of peptide units confined to the first turns of helices. The contribution of helix macrodipoles appears insignificant.

Interdomain salt bridges modulate ligand-induced domain motion of the sulfate receptor protein for active transport

The refined crystal structure of the liganded form of the Salmonella typhimurium sulfate-binding protein, a periplasmic receptor of active transport, is made up of two globular domains bisected by a deep cleft wherein the dehydrated sulfate is completely engulfed and bound by hydrogen bonds and van der Waals' forces. Two salt bridges (between Glu15 and Arg174 and between Asp68 and Arg134) span the cleft opening. To elucidate the role of the inter-domain salt bridges in the ligand-induced domain motion, the acidic residues were changed (singly and together) to their corresponding amide side-chains by site-directed mutagenesis of the recombinant Escherichia coli sulfate-binding protein. Rapid kinetics and equilibrium measurements of sulfate binding to the purified mutant proteins demonstrate that these salt bridges stabilize the closed liganded form of the receptor and modulate the rate of cleft opening. Our results have new implications in understanding the dynamics of many other multidomain proteins that undergo similar large-scale domain motions.

A nonconservative serine to cysteine mutation in the sulfate-binding protein, a transport receptor

Serine 130 is one of seven residues that form a total of seven hydrogen bonds with the sulfate completely sequestered deep in the cleft between the two lobes of the bilobate sulfate-binding protein from Salmonella typhimurium. This residue has been replaced with Cys, Ala, and Gly by site-directed mutagenesis in an Escherichia coli expression system. Replacement with the isosteric Cys caused a 3200-fold decrease in the sulfate-binding activity relative to the wild-type activity, whereas replacement with Ala and Gly resulted in only 100- and 15-fold decreases, respectively. The effect of the Cys substitution is attributed largely to steric effect, whereas the Gly substitution more nearly reflects the loss of one hydrogen bond to the bound sulfate with a strength of only 1.6 kilocalories per mole.

Engineered interdomain disulfide in the periplasmic receptor for sulfate transport reduces flexibility. Site-directed mutagenesis and ligand-binding studies

Using recombinant DNA techniques, an Escherichia coli periplasmic sulfate receptor or sulfate-binding protein involved in active transport has been overexpressed and characterized. This protein is essentially identical in size, sequence, antigenicity, and ligand affinity and specificity to the sulfate receptor from Salmonella typhimurium whose crystal structure has been refined at 2 A resolution. The dehydrated sulfate is bound in the deep cleft between the two lobes of the bilobate protein. Using the structure of the S. typhimurium as a guide, three site-directed mutants (Ser129Cys, Gly46Cys, and Ser129Cys/Gly46Cys) have been made. In the Cys129/Cys46 mutant the disulfide has been successfully introduced across the opening of the ligand-binding site cleft of the E. coli sulfate-binding protein. The dissociation of sulfate from the double mutant protein is very slow under oxidizing conditions and increases more than 200-fold when reducing agent is added. This effect is attributed to a loss of interdomain structural flexibility in the presence of the disulfide, and underscores the importance of protein conformational change in binding protein function.

Effect of amino acid alterations in the tryptophan-binding site of the trp repressor

Mutations in the tryptophan-binding site of the trp repressor have been generated using site-directed mutagenesis. The selection of sites for alteration was based on the three-dimensional x-ray crystallographic structure (Schevitz, R. W., Otwinowski, Z., Joachimiak, A., Lawson, C. L., and Sigler, P. B. (1985) Nature 317, 782-786). The changes generated include Thr-44 to Ala (T44A), Arg-54 to Leu (R54L), Arg-54 to Lys (R54K), Arg-84 to Leu (R84L), and Arg-84 to Lys (R84K). The mutant proteins were purified and characterized in detail for their binding properties. Both tryptophan and operator DNA affinities for all five mutants were decreased. The R84L, R54K, and R54L mutants exhibited increases in Kd for operator DNA relative to wild-type repressor ranging from approximately 10(3) to approximately 10(4), while R84K and T44A exhibited increases of 10- to 100-fold. This diminution in DNA binding activity derives at least in part from diminished affinity for tryptophan, although decreased affinity for nonspecific DNA was also observed for these mutant proteins. Tryptophan binding was not detectable by equilibrium dialysis for most of the mutant proteins, but this activity was measurable for several of the altered proteins by monitoring the fluorescence decrease associated with the displacement of 1-anilino-8-naphthalenesulfonate from the tryptophan-binding site (Chou, W.-Y., and Matthews, K. S. (1989) J. Biol. Chem. 264, 18314-18319). These measurements revealed that tryptophan bound to R84K, T44A, and R84L repressors with Kd values 1.5- to 13-fold higher than that for wild-type repressor. It was not possible to detect tryptophan binding to R54K and R54L even using the fluorescence assay. Circular dichroism spectra demonstrated that the mutants and the wild-type repressor possess similar secondary structural features. The results of this selected substitution in the tryptophan-binding site are readily interpreted based on the x-ray structural analysis.