Activation mechanism of the µ-opioid receptor by an allosteric modulator.

Allosteric modulators of G-protein-coupled receptors (GPCRs) enhance signaling by binding to GPCRs concurrently with their orthosteric ligands, offering a novel approach to overcome the efficacy limitations of conventional orthosteric ligands. However, the structural mechanism by which allosteric modulators mediate GPCR signaling remains largely unknown. Here, to elucidate the mechanism of µ-opioid receptor (MOR) activation by allosteric modulators, we conducted solution NMR analyses of MOR by monitoring the signals from methionine methyl groups. We found that the intracellular side of MOR exists in an equilibrium between three conformations with different activities. Interestingly, the populations in the equilibrium determine the apparent signaling activity of MOR. Our analyses also revealed that the equilibrium is not fully shifted to the conformation with the highest activity even in the full agonist-bound state, where the intracellular half of TM6 is outward-shifted. Surprisingly, an allosteric modulator for MOR, BMS-986122, shifted the equilibrium toward the conformation with the highest activity, leading to the increased activity of MOR in the full agonist-bound state. We also determined that BMS-986122 binds to a cleft in the transmembrane region around T162 on TM3. Together, these results suggest that BMS-986122 binding to TM3 increases the activity of MOR by rearranging the direct interactions of TM3 and TM6, thus stabilizing TM6 in the outward-shifted position which is favorable for G-protein binding. These findings shed light on the rational developments of novel allosteric modulators that activate GPCRs further than orthosteric ligands alone and pave the way for next-generation GPCR-targeting therapeutics.

Paip2A inhibits translation by competitively binding to the RNA recognition motifs of PABPC1 and promoting its dissociation from the poly(A) tail.

Eukaryotic mRNAs possess a poly(A) tail at their 3'-end, to which poly(A)-binding protein C1 (PABPC1) binds and recruits other proteins that regulate translation. Enhanced poly(A)-dependent translation, which is also PABPC1 dependent, promotes cellular and viral proliferation. PABP-interacting protein 2A (Paip2A) effectively represses poly(A)-dependent translation by causing the dissociation of PABPC1 from the poly(A) tail; however, the underlying mechanism remains unknown. This study was conducted to investigate the functional mechanisms of Paip2A action by characterizing the PABPC1-poly(A) and PABPC1-Paip2A interactions. Isothermal titration calorimetry and NMR analyses indicated that both interactions predominantly occurred at the RNA recognition motif (RRM)2-RRM3 regions of PABPC1, which have comparable affinities for poly(A) and Paip2A (dissociation constant, Kd = 1 nM). However, the Kd values of isolated RRM2 were 200 and 4 µM in their interactions with poly(A) and Paip2A, respectively; Kd values of 5 and 1 µM were observed for the interactions of isolated RRM3 with poly(A) and Paip2A, respectively. NMR analyses also revealed that Paip2A can bind to the poly(A)-binding interfaces of the RRM2 and RRM3 regions of PABPC1. Based on these results, we propose the following functional mechanism for Paip2A: Paip2A initially binds to the RRM2 region of poly(A)-bound PABPC1, and RRM2-anchored Paip2A effectively displaces the RRM3 region from poly(A), resulting in dissociation of the whole PABPC1 molecule. Together, our findings provide insight into the translation repression effect of Paip2A and may aid in the development of novel anticancer and/or antiviral drugs.

Structural equilibrium underlying ligand-dependent activation of ß2-adrenoreceptor.

G-protein-coupled receptors (GPCRs) are seven-transmembrane proteins mediating cellular signals in response to extracellular stimuli. Although three-dimensional structures showcase snapshots that can be sampled in the process and nuclear magnetic resonance detects conformational equilibria, the mechanism by which agonist-activated GPCRs interact with various effectors remains elusive. Here, we used paramagnetic nuclear magnetic resonance for leucine amide resonances to visualize the structure of ß2-adrenoreceptor in the full agonist-bound state, without thermostabilizing mutations abolishing its activity. The structure exhibited a unique orientation of the intracellular half of the transmembrane helix 6, forming a cluster of G-protein-interacting residues. Furthermore, analyses of efficacy-dependent chemical shifts of the residues near the pivotal PIF microswitch identified an equilibrium among three conformations, including one responsible for the varied signal level in each ligand-bound state. Together, these results provide a structural basis for the dynamic activation of GPCRs and shed light on GPCR-mediated signal transduction.

Assessment of surgical outcomes of off-clamp open partial nephrectomy without renorrhaphy for ≥T1b renal tumours.

PURPOSE: To assess the surgical outcomes of off-clamp open partial nephrectomy without renorrhaphy. In the era of robot-assisted surgeries, open partial nephrectomy remains a surgical option for ≥ T1b renal tumours. Although the necessity of renal pedicle clamping and renorrhaphy in open partial nephrectomy for larger tumours remains to be discussed, reports on this issue are rare. METHODS: Twenty-seven open partial nephrectomies for ≥ T1b renal tumours were performed without renal pedicle clamping or renorrhaphy. A soft coagulation system was used to control bleeding from the resection bed. Surgical results, complications, and predictors of perioperative estimated glomerular filtration rate (eGFR) preservation at 1 month and 3 months after surgery were analysed. RESULTS: The median estimated volume of blood loss was 420 mL. The rates of perioperative eGFR preservation were 88.9 and 87.3% at 1 and 3 months after surgery, respectively. Tumour size was an independent predictor of perioperative eGFR preservation at 1 month after surgery, whereas age and exophytic/endophytic properties of the tumour were independent predictors of perioperative eGFR preservation at 3 months after surgery. CONCLUSION: Open partial nephrectomy without renal pedicle clamping or renorrhaphy could be safely performed for ≥ T1b renal tumours, even when tumours were entirely endophytic and located close to the renal pedicle. Mild perioperative eGFR reduction was observed. Although surgical indications should be carefully considered in these cases, off-clamp open partial nephrectomy without renorrhaphy is a feasible procedure for patients with ≥ T1b renal tumours.

[The Optimal Start Timing of Helical Scan for Coronary CT Angiography and CT Myocardial Perfusion Scan Using 64-MDCT].

PURPOSE: Ichihara et al. (Fujita Med J 2015; 1(1): 9-14) developed a method to simultaneously obtain both coronary computed tomography (CT) angiography and CT myocardial perfusion (CTP) using 64-multi detector CT (MDCT). An input-function (time enhancement curve, TEC) of the ascending aorta (Ao) and myocardial CT density are necessary to calculate absolute myocardial blood flow (ml/g/min) using a two-compartment model. Helical scan starting timing is important to capture the peak (P) of Ao time enhancement curve (TEC). The purpose is to search the optimal timing of starting helical scan to capture the P. METHODS: We performed 14 CTPs using Definition AS+ (SIEMENS). A dynamic scan at the Ao level was started at 7 s after contrast injection and helical scan was started at various trigger on bolus tracking. Definition AS+ needs 2 s (other scanner may need 4 s) for changing from a dynamic to helical scan mode. We created TECs of pulmonary artery (PA) and Ao using the fifth function fitting. We measured the time from trigger point to the P (t200, t250, t300 and tCP). RESULTS: Mean t200, t250, t300 and tCP were 9.1±1.9, 7.9±2.0, 6.6±1.9 and 3.9±1.2 s, respectively. In additional other 16 CTP studies using the cross point method, we can capture the P in all (100%) examinations. CONCLUSION: Scan starting at the cross point is best for Definition AS+, and the Ao=300 HU may be best for other scanner that needs 4 s for changing scan mode to obtain a fine input function for calculating absolute myocardial blood flow.

Deuteration and selective labeling of alanine methyl groups of ß2-adrenergic receptor expressed in a baculovirus-insect cell expression system.

G protein-coupled receptors (GPCRs) exist in equilibrium between multiple conformations, and their populations and exchange rates determine their functions. However, analyses of the conformational dynamics of GPCRs in lipid bilayers are still challenging, because methods for observations of NMR signals of large proteins expressed in a baculovirus-insect cell expression system (BVES) are limited. Here, we report a method to incorporate methyl-13C1H3-labeled alanine with > 45% efficiency in highly deuterated proteins expressed in BVES. Application of the method to the NMR observations of ß2-adrenergic receptor in micelles and in nanodiscs revealed the ligand-induced conformational differences throughout the transmembrane region of the GPCR.

Insulin resistance increases the risk of incident type 2 diabetes mellitus in patients with non-alcoholic fatty liver disease.

AIM: Type 2 diabetes mellitus (T2DM) is a major complication of patients with non-alcoholic fatty liver disease (NAFLD). The aim of this retrospective study is to determine the risk factors for development of T2DM in patients with biopsy-proven NAFLD. METHODS: One hundred and sixty two consecutive patients with biopsy-proven NAFLD who received a 75-g oral glucose tolerance test were enrolled as the total cohort. Among them, we analyzed 89 patients without T2DM diagnosed by oral glucose tolerance test to estimate the cumulative rate for development of T2DM as the follow-up cohort. RESULTS: Of 162 patients, the glucose tolerance pattern were DM in 45 patients (27.8%), impaired glucose tolerance in 68 (42.0%), and normal glucose tolerance in 49 (30.2%). Patients with NAFL tended to be more likely to have normal glucose tolerance than those with non-alcoholic steatohepatitis (NASH). The serum levels of pre- and post-load insulin were significantly higher in the NASH group. Of 89 patients without T2DM, 13 patients newly developed T2DM during a follow-up period of 5.2 years. The cumulative rate of T2DM incidence was 8.8% at the end of the 5th year and 23.4% at the end of the 10th year. Multivariate analysis identified homeostasis model of assessment - insulin resistance (≥3.85, hazard ratio 40.1, P = 0.033) as an independent risk factor for development of T2DM. CONCLUSIONS: Patients with NASH have an underlying potential of glucose intolerance. In NAFLD patients, insulin resistance is the most important risk factor for the incidence of T2DM. Appropriate therapy against insulin resistance could be needed for patients with NAFLD to prevent development of T2DM.

Effect of 12-week dulaglutide therapy in Japanese patients with biopsy-proven non-alcoholic fatty liver disease and type 2 diabetes mellitus.

AIMS: No pharmacological therapies have been established for non-alcoholic fatty liver disease (NAFLD) with type 2 diabetes mellitus (T2DM). The aim of this retrospective study is to evaluate the efficacy and safety of dulaglutide, a novel glucagon-like peptidase-1 receptor agonist, in Japanese NAFLD patients with T2DM. METHODS: Fifteen biopsy-proven NAFLD patients with T2DM refractory to diet intervention who received once weekly dulaglutide 0.75 mg for 12 weeks were retrospectively enrolled after exclusion of two patients by 12 weeks. In five patients, transient elastography and body composition were also evaluated before and after the treatment. RESULTS: Not only body weight and hemoglobin A1c but also transaminase activities were significantly decreased after the 12-week therapy with dulaglutide. Total body fat mass and liver stiffness measurement also decreased after the treatment. CONCLUSION: Dulaglutide, a new glucagon-like peptidase-1 receptor agonist, could be a novel promising agent for the treatment for NAFLD patients with T2DM due to its efficacy in body weight reduction, the nature of weekly injection, and patient preference. Prospective randomized controlled trials are warranted to confirm this impact of dulaglutide on NAFLD with T2DM.

Effect of sodium glucose cotransporter 2 inhibitor on liver function tests in Japanese patients with non-alcoholic fatty liver disease and type 2 diabetes mellitus.

AIM: No pharmacological therapies have been established for non-alcoholic fatty liver disease (NAFLD). Sodium glucose cotransporter 2 inhibitor (SGLT2I) was developed for the treatment of adults with type 2 diabetes mellitus (T2DM). The aim of this retrospective study is to evaluate the efficacy of SGLT2I in NAFLD patients with T2DM. METHODS: Twenty-four biopsy-proven NAFLD patients with T2DM who received SGLT2I for 24 weeks were retrospectively enrolled as the SGLT2I group. Another 21 NAFLD patients with T2DM treated with dipeptidyl peptidase-4 inhibitor (DPP4I) for 24 weeks were selected as the DPP4I group. Clinical data were evaluated at baseline and at 4, 12, and 24 weeks. Seventeen patients in the SGLT2I group were evaluated by body composition before and after therapy. RESULTS: Not only body weight and hemoglobin A1c but also transaminase activities were significantly decreased in the SGLT2I group. Reductions in transaminase activities were similar between SGLT2I and DPP4I groups. In the SGLT2I group, body mass index and fasting plasma glucose also decreased after the treatment. CONCLUSION: Sodium glucose cotransporter 2 inhibitor can be a novel promising agent for the treatment for NAFLD patients with T2DM. Prospective randomized controlled trials are warranted to confirm this efficacy of SGLT2I on NAFLD with T2DM.

Development of hepatocellular carcinoma in Japanese patients with biopsy-proven non-alcoholic fatty liver disease: Association between PNPLA3 genotype and hepatocarcinogenesis/fibrosis progression.

AIM: Some patients with non-alcoholic fatty liver disease (NAFLD) develop hepatocellular carcinoma (HCC). Patatin-like phospholipase domain containing 3 (PNPLA3) rs738409 (encoding the I148M variant) has been associated with advanced fibrosis and HCC. We determined the risk factors for HCC, including the PNPLA3 rs738409 polymorphism, in Japanese patients with biopsy-proven NAFLD. METHODS: In this retrospective cohort study, we analyzed hepatocarcinogenesis in 238 patients. PNPLA3 rs738409 genotype was determined by allelic discrimination in 130 patients. Among them, 86 patients who were followed up for >5 years and without liver cirrhosis were analyzed to clarify the relationship between PNPLA3 genotype and long-term changes in biomarkers. RESULTS: Of 238 patients, PNPLA3 genotype frequencies were: CC, 0.14; CG, 0.46; and GG, 0.40. During a follow-up period of 6.1 years, 10 patients (4.2%) with non-alcoholic steatohepatitis developed HCC. The cumulative rate of HCC was 1.9% at the end of the 5th year and 8.3% at the end of the 10th year. Multivariate analysis identified PNPLA3 genotype GG (hazard ratio, 6.36; P = 0.019) and fibrosis stage (fibrosis stage 3/4; hazard ratio, 24.4; P = 0.011) as predictors of HCC development. In the long follow-up cohort, a larger reduction in platelet count was found in the GG group (P = 0.032) despite a larger reduction in alanine aminotransferase (P = 0.023) compared to that in the CC/CG group. CONCLUSIONS: In Japanese patients with NAFLD, severe fibrosis and PNPLA3 GG genotype were predictors of HCC development, independent of other known risk factors. Patients with the PNPLA3 GG genotype have the potential for a decreased platelet count, even when alanine aminotransferase levels are well controlled.

Identification of natural rubber degradation gene in Rhizobacter gummiphilus NS21.

A Gram-negative rubber-degrading bacterium, Rhizobacter gummiphilus NS21 grew and produced aldehyde metabolites on a deproteinized natural rubber (DPNR)-overlay agar medium forming a clearing zone. A transposon-insertion mutant, which had lost the ability to degrade DPNR, was isolated to identify the rubber degradation genes. Sequencing analysis indicated that the transposon was inserted into a putative oxygenase gene, latA. The deduced amino acid sequence of latA has 36% identity with that of roxA, which encodes a rubber oxygenase of Xanthomonas sp. strain 35Y. Phylogenetic analysis revealed that LatA constitutes a distinct group from RoxA. Heterologous expression in a Methylibium host and deletion analysis of latA indicated that the latA product is responsible for the depolymerization of DPNR. The quantitative reverse transcription-PCR analysis indicated that the transcription of latA is induced during the growth on DPNR. These results strongly suggest that latA is directly involved in the degradation of rubber in NS21.

An RNA-binding Protein, Lin28, Recognizes and Remodels G-quartets in the MicroRNAs (miRNAs) and mRNAs It Regulates.

Lin28 is an evolutionarily conserved RNA-binding protein that inhibits processing of pre-let-7 microRNAs (miRNAs) and regulates translation of mRNAs that control developmental timing, pluripotency, metabolism, and tumorigenesis. The RNA features that mediate Lin28 binding to the terminal loops of let-7 pre-miRNAs and to Lin28-responsive elements (LREs) in mRNAs are not well defined. Here we show that Lin28 target datasets are enriched for RNA sequences predicted to contain stable planar structures of 4 guanines known as G-quartets (G4s). The imino NMR spectra of pre-let-7 loops and LREs contain resonances characteristic of G4 hydrogen bonds. These sequences bind to a G4-binding fluorescent dye, N-methyl-mesoporphyrin IX (NMM). Mutations and truncations in the RNA sequence that prevent G4 formation also prevent Lin28 binding. The addition of Lin28 to a pre-let-7 loop or an LRE reduces G4 resonance intensity and NMM binding, suggesting that Lin28 may function to remodel G4s. Further, we show that NMM inhibits Lin28 binding. Incubation of a human embryonal carcinoma cell line with NMM reduces its stem cell traits. In particular it increases mature let-7 levels, decreases OCT4, HMGA1, CCNB1, CDK4, and Lin28A protein, decreases sphere formation, and inhibits colony formation. Our results suggest a previously unknown structural feature of Lin28 targets and a new strategy for manipulating Lin28 function.

γ-Resorcylate catabolic-pathway genes in the soil actinomycete Rhodococcus jostii RHA1.

The Rhodococcus jostii RHA1 gene cluster required for γ-resorcylate (GRA) catabolism was characterized. The cluster includes tsdA, tsdB, tsdC, tsdD, tsdR, tsdT, and tsdX, which encode GRA decarboxylase, resorcinol 4-hydroxylase, hydroxyquinol 1,2-dioxygenase, maleylacetate reductase, an IclR-type regulator, a major facilitator superfamily transporter, and a putative hydrolase, respectively. The tsdA gene conferred GRA decarboxylase activity on Escherichia coli. Purified TsdB oxidized NADH in the presence of resorcinol, suggesting that tsdB encodes a unique NADH-specific single-component resorcinol 4-hydroxylase. Mutations in either tsdA or tsdB resulted in growth deficiency on GRA. The tsdC and tsdD genes conferred hydroxyquinol 1,2-dioxygenase and maleylacetate reductase activities, respectively, on E. coli. Inactivation of tsdT significantly retarded the growth of RHA1 on GRA. The growth retardation was partially suppressed under acidic conditions, suggesting the involvement of tsdT in GRA uptake. Reverse transcription-PCR analysis revealed that the tsd genes constitute three transcriptional units, the tsdBADC and tsdTX operons and tsdR. Transcription of the tsdBADC and tsdTX operons was induced during growth on GRA. Inactivation of tsdR derepressed transcription of the tsdBADC and tsdTX operons in the absence of GRA, suggesting that tsd gene transcription is negatively regulated by the tsdR-encoded regulator. Binding of TsdR to the tsdR-tsdB and tsdT-tsdR intergenic regions was inhibited by the addition of GRA, indicating that GRA interacts with TsdR as an effector molecule.

Human placental extract treatment for non-alcoholic steatohepatitis non-responsive to lifestyle intervention: A pilot study.

AIM: No pharmacological therapies have been established for non-alcoholic steatohepatitis (NASH), which can lead to liver-related mortality. Human placental extract (HPE), which has anti-inflammatory effects, has been expected to be a promising treatment for chronic liver disease. This pilot study was conducted to evaluate the efficacy of HPE for biopsy-diagnosed NASH. METHODS: After a lifestyle intervention for 12 weeks, 10 subjects with abnormal alanine aminotransferase (≥30 IU/L) and biopsy-proven NASH (Non-Alcoholic Fatty Liver Disease Activity Score [NAS], ≥4) received i.m. injections of HPE (Laennec) at a dose of 4 mL/day twice per week for 24 weeks, and seven of them underwent a second liver biopsy after the treatment. Liver biopsies were scored for NAS and fibrosis. Histological response was defined as a decrease of 2 points or more in NAS and no increase in fibrosis. RESULTS: Serum transaminase activities were significantly lower at 8 weeks compared with pretreatment levels in nine patients who continued treatment for 24 weeks. One patient refused to continue the treatment soon after starting therapies. In seven patients undergoing post-treatment biopsies, NAS (mean [standard deviation]) mildly decreased from 5.29 (0.95) to 4.00 (1.83) without reaching statistical significance (P = 0.078). Histological response was observed in all three obese patients and in only one of four non-obese ones. No significant changes were observed in body mass index, lipid profiles and diabetic control/insulin resistance. CONCLUSION: In NASH patients who received HPE treatment, significant reductions in serum liver enzymes were obtained after 8 weeks. Histological efficacy may be better in obese patients than in non-obese ones.

Serum alanine aminotransferase predicts the histological course of non-alcoholic steatohepatitis in Japanese patients.

AIM: Some cases with non-alcoholic fatty liver disease (NAFLD), particularly non-alcoholic steatohepatitis (NASH), can ultimately progress to liver cirrhosis. However, studies to clarify factors predictive of histological change in patients with NASH remain scarce. Our aim is to determine predictors of histological progression in Japanese patients with biopsy-proven NASH. METHODS: This retrospective cohort study enrolled 52 patients with NASH who underwent serial liver biopsies. Histological evaluation included NAFLD activity score (NAS) and liver fibrosis. The median interval between initial and second liver biopsies was 968 days. An alanine aminotransferase (ALT) response was defined as a decrease of 30% or more from baseline. RESULTS: Of 52 patients, NAS was ameliorated in 30.8%, deteriorated in 30.8% and remained unchanged in 38.4%. Liver fibrosis was improved in 25.0% of patients, progressed in 25.0% and remained stable in 50.0%. Multivariate analysis identified ALT non-response as a predictor of deterioration of NAS (hazard ratio [HR], 5.85; P = 0.031) and progression of liver fibrosis (HR, 4.50; P = 0.029). The mean annual rate of fibrosis was 0.002 stages/year overall, increasing to 0.15 stages/year in ALT non-responders. CONCLUSION: A lack of reduction in serum ALT level by at least 30% from baseline was a predictor for histological progression in patients with NASH. Serum ALT level is a better predictor of histological change than insulin resistance or bodyweight and can be a valid index in treatment. Serum ALT should be strictly controlled to prevent liver histological progression in patients with NASH.

Lower levels of insulin-like growth factor-1 standard deviation score are associated with histological severity of non-alcoholic fatty liver disease.

AIM: Growth hormone (GH) deficiency may be associated with histological progression of non-alcoholic fatty liver disease (NAFLD) which includes non-alcoholic fatty liver (NAFL) and non-alcoholic steatohepatitis (NASH). Insulin-like growth factor 1 (IGF-1) is mainly produced by hepatocytes and its secretion is stimulated by GH. Our aim was to determine whether more histologically advanced NAFLD is associated with low circulating levels of IGF-1 in Japanese patients. METHODS: Serum samples were obtained in 199 Japanese patients with biopsy-proven NAFLD and in 2911 sex- and age-matched healthy people undergoing health checkups. The serum levels of IGF-1 were measured using a commercially available immunoradiometric assay. The standard deviation scores (SDS) of IGF-1 according to age and sex were also calculated in NAFLD patients. RESULTS: The serum IGF-1 levels in NAFLD patients were significantly lower (median, 112 ng/mL) compared with the control population (median, 121 ng/mL, P < 0.0001). IGF-1 SDS less than -2.0 SD from median were found in 11.6% of 199 patients. NASH patients exhibited significantly lower levels of IGF-1 SDS (n = 130; median, -0.7) compared with NAFL patients (n = 69; median, -0.3; P = 0.026). The IGF-1 SDS values decreased significantly with increasing lobular inflammation (P < 0.001) and fibrosis (P < 0.001). In multiple regressions, the association between the IGF-1 SDS values and the severity of NAFLD persisted after adjusting for age, sex and insulin resistance. CONCLUSION: Low levels of circulating IGF-1 may have a role in the development of advanced NAFLD, independent of insulin resistance. Supplementation with GH/IGF-1 may be a candidate for the treatment of NASH.

Biological role of the two overlapping poly(A)-binding protein interacting motifs 2 (PAM2) of eukaryotic releasing factor eRF3 in mRNA decay.

Eukaryotic releasing factor GSPT/eRF3 mediates translation termination-coupled mRNA decay via interaction with a cytosolic poly(A)-binding protein (PABPC1). A region of eRF3 containing two overlapping PAM2 (PABPC1-interacting motif 2) motifs is assumed to bind to the PABC domain of PABPC1, on the poly(A) tail of mRNA. PAM2 motifs are also found in the major deadenylases Caf1-Ccr4 and Pan2-Pan3, whose activities are enhanced upon PABPC1 binding to these motifs. Their deadenylase activities are regulated by eRF3, in which two overlapping PAM2 motifs competitively prevent interaction with PABPC1. However, it is unclear how these overlapping motifs recognize PABC and regulate deadenylase activity in a translation termination-coupled manner. We used a dominant-negative approach to demonstrate that the N-terminal PAM2 motif is critical for eRF3 binding to PABPC1 and that both motifs are required for function. Isothermal titration calorimetry (ITC) and NMR analyses revealed that the interaction is in equilibrium between the two PAM2-PABC complexes, where only one of the two overlapping PAM2 motifs is PABC-bound and the other is PABC-unbound and partially accessible to the other PABC. Based on these results, we proposed a biological role for the overlapping PAM2 motifs in the regulation of deadenylase accessibility to PABPC1 at the 3' end of poly(A).

Structural and dynamic insights into the activation of the µ-opioid receptor by an allosteric modulator.

G-protein-coupled receptors (GPCRs) play pivotal roles in various physiological processes. These receptors are activated to different extents by diverse orthosteric ligands and allosteric modulators. However, the mechanisms underlying these variations in signaling activity by allosteric modulators remain largely elusive. Here, we determine the three-dimensional structure of the µ-opioid receptor (MOR), a class A GPCR, in complex with the Gi protein and an allosteric modulator, BMS-986122, using cryogenic electron microscopy. Our results reveal that BMS-986122 binding induces changes in the map densities corresponding to R1673.50 and Y2545.58, key residues in the structural motifs conserved among class A GPCRs. Nuclear magnetic resonance analyses of MOR in the absence of the Gi protein reveal that BMS-986122 binding enhances the formation of the interaction between R1673.50 and Y2545.58, thus stabilizing the fully-activated conformation, where the intracellular half of TM6 is outward-shifted to allow for interaction with the Gi protein. These findings illuminate that allosteric modulators like BMS-986122 can potentiate receptor activation through alterations in the conformational dynamics in the core region of GPCRs. Together, our results demonstrate the regulatory mechanisms of GPCRs, providing insights into the rational development of therapeutics targeting GPCRs.

Functional equilibrium of the KcsA structure revealed by NMR.

KcsA is a tetrameric K(+) channel that is activated by acidic pH. Under open conditions of the helix bundle crossing, the selectivity filter undergoes an equilibrium between permeable and impermeable conformations. Here we report that the population of the permeable conformation (p(perm)) positively correlates with the tetrameric stability and that the population in reconstituted high density lipoprotein, where KcsA is surrounded by the lipid bilayer, is lower than that in detergent micelles, indicating that dynamic properties of KcsA are different in these two media. Perturbation of the membrane environment by the addition of 1-3% 2,2,2-trifluoroethanol increases p(perm) and the open probability, revealed by NMR and single-channel recording analyses. These results demonstrate that KcsA inactivation is determined not only by the protein itself but also by the surrounding membrane environments.

Spatial distribution of cytoplasmic domains of the Mg(2+)-transporter MgtE, in a solution lacking Mg(2+), revealed by paramagnetic relaxation enhancement.

MgtE is a prokaryotic Mg(2+) transporter that controls cellular Mg(2+) concentrations. We previously reported crystal structures of the cytoplasmic region of MgtE, consisting of 2 domains, that is, N and CBS, in the Mg(2+)-free and Mg(2+)-bound forms. The Mg(2+)-binding sites lay at the interface of the 2 domains, making the Mg(2+)-bound form compact and globular. In the Mg(2+)-free structure, however, the domains are far apart, and the Mg(2+)-binding sites are destroyed. Therefore, it is unclear how Mg(2+)-free MgtE changes its conformation to accommodate Mg(2+) ions. Here, we used paramagnetic relaxation enhancement (PRE) to characterize the relative orientation of the N and CBS domains in the absence of Mg(2+) in solution. When the residues on the surface of the CBS domain were labeled with nitroxide tags, significant PRE effects were observed for the residues in the N domain. No single structure satisfied the PRE profiles, suggesting that the N and CBS domains are not fixed in a particular orientation in solution. We then conducted ensemble simulated annealing calculations in order to obtain the atomic probability density and visualize the spatial distribution of the N domain in solution. The results indicate that the N domain tends to occupy the space near its position in the Mg(2+)-bound crystal structure, facilitating efficient capture of Mg(2+) with increased intracellular Mg(2+) concentration, which is necessary to close the gate.