Structural basis for the auxin-induced transcriptional regulation by Aux/IAA17.

Auxin is the central hormone that regulates plant growth and organ development. Transcriptional regulation by auxin is mediated by the auxin response factor (ARF) and the repressor, AUX/IAA. Aux/IAA associates with ARF via domain III-IV for transcriptional repression that is reversed by auxin-induced Aux/IAA degradation. It has been known that Aux/IAA and ARF form homo- and hetero-oligomers for the transcriptional regulation, but what determines their association states is poorly understood. Here we report, to our knowledge, the first solution structure of domain III-IV of Aux/IAA17 (IAA17), and characterize molecular interactions underlying the homotypic and heterotypic oligomerization. The structure exhibits a compact ß-grasp fold with a highly dynamic insert helix that is unique in Aux/IAA family proteins. IAA17 associates to form a heterogeneous ensemble of front-to-back oligomers in a concentration-dependent manner. IAA17 and ARF5 associate to form homo- or hetero-oligomers using a common scaffold and binding interfaces, but their affinities vary significantly. The equilibrium dissociation constants (KD) for homo-oligomerization are 6.6 µM and 0.87 µM for IAA17 and ARF5, respectively, whereas hetero-oligomerization reveals a ∼ 10- to ∼ 100-fold greater affinity (KD = 73 nM). Thus, individual homo-oligomers of IAA17 and ARF5 spontaneously exchange their subunits to form alternating hetero-oligomers for transcriptional repression. Oligomerization is mainly driven by electrostatic interactions, so that charge complementarity at the interface determines the binding affinity. Variable binding affinity by surface charge modulation may effectively regulate the complex interaction network between Aux/IAA and ARF family proteins required for the transcriptional control of auxin-response genes.

An unusual protein-protein interaction through coupled unfolding and binding.

Aptides, a novel class of high-affinity peptides, recognize diverse molecular targets with high affinity and specificity. The solution structure of the aptide APT specifically bound to fibronectin extradomain B (EDB), which represents an unusual protein-protein interaction that involves coupled unfolding and binding, is reported. APT binding is accompanied by unfolding of the C-terminal ß strand of EDB, thereby permitting APT to interact with the freshly exposed hydrophobic interior surfaces of EDB. The ß-hairpin scaffold of APT drives the interaction by a ß-strand displacement mechanism, such that an intramolecular ß sheet is replaced by an intermolecular ß sheet. The unfolding of EDB perturbs the tight domain association between EDB and FN8 of fibronectin, thus highlighting its potential use as a scaffold that switches between stretched and bent conformations.

Soraphinol C, a new free-radical scavenger from Sorangium cellulosum.

A new compound named soraphinol C (1) was isolated from myxobacteria Sorangium cellulosum KM1001 a soil isolate, together with a structurally related known compound, 4-hydroxysattabacin (2). These compounds were isolated by silica gel column chromatography and recycling preparative HPLC, consecutively. The structures of the compounds were determined on the basis of combined spectroscopic analyses. Compounds 1 and 2 exhibited antioxidant activity as a radical scavenger in the experiment using a hydrophilic free-radical initiator, 2,2'-azobis(2-amidinopropane)dihydrochloride with ORAC values of 0.956 and 0.617, respectively.

Early prediction of myocardial viability after acute myocardial infarction by two-dimensional speckle tracking imaging.

BACKGROUND: Identifying the transmural extent of myocardial necrosis and the degree of myocardial viability in acute myocardial infarction (AMI) is important clinically. The aim of this study was to assess myocardial viability using two-dimensional speckle tracking imaging (2D-STI) in patients with AMI. METHODS: 2D-STI was performed at initial presentation, three days, and six months after primary percutaneous coronary intervention (PCI) in 30 patients with AMI, who had a left anterior descending coronary artery (LAD) culprit lesion. In addition, 20 patients who had minimal stenotic lesions (< 30% stenosis) on coronary angiography were also included in the control group. At six months dobutamine echocardiography was performed for viability assessment in seven segments of the LAD territory. According to the recovery of wall motion abnormality, segments were classified as viable or non-viable. RESULTS: A total of 131 segments were viable, and 44 were nonviable. Multivariate analysis revealed significant differences between the viable and nonviable segments in the peak systolic strain, the peak systolic strain rate at initial presentation, and peak systolic strain rate three days after primary PCI. Among these, the initial peak systolic strain rate had the highest predictive value for myocardial viability (hazard ratio: 31.22, P < 0.01). CONCLUSIONS: 2D-STI is feasible for assessing myocardial viability, and the peak systolic strain rate might be the most reliable predictor of myocardial viability in patients with AMI.

Crystal structure of toxin HP0892 from Helicobacter pylori with two Zn(II) at 1.8 Å resolution.

Antibiotic resistance and microorganism virulence have been consistently exhibited by bacteria and archaea, which survive in conditions of environmental stress through toxin-antitoxin (TA) systems. The HP0892-HP0893 TA system is one of the two known TA systems belonging to Helicobacter pylori. The antitoxin, HP0893, binds and inhibits the HP0892 toxin and regulates the transcription of the TA operon. Here, we present the crystal structure of the zinc-bound HP0892 toxin at 1.8 Å resolution. Reorientation of residues at the mRNase active site was shown. The involved residues, namely E58A, H86A, and H58A/ H60A, were mutated and the binding affinity was monitored by ITC studies. Through the structural difference between the apo and the metal-bound state, and using a homology modeling tool, the involvement of the metal ion in mRNase active site could be identified. The most catalytically important residue, His86, reorients itself to exhibit RNase activity. His47, Glu58, and His60 are involved in metal binding where Glu58 acts as a general base and His47 and His60 may also act as a general acid in enzymatic activity. Glu58 and Asp64 are involved in substrate binding and specific sequence recognition. Arg83 is involved in phosphate binding and stabilization of the transition state, and Phe90 is involved in base packing and substrate orientation.

Active site phosphoryl groups in the biphosphorylated phosphotransferase complex reveal dynamics in a millisecond time scale.

The N-terminal domain of Enzyme I (EIN) and phosphocarrier HPr can form a biphosphorylated complex when they are both phosphorylated by excess cellular phosphoenolpyruvate. Here we show that the electrostatic repulsion between the phosphoryl groups in the biphosphorylated complex results in characteristic dynamics at the active site in a millisecond time scale. The dynamics is localized to phospho-His15 and the stabilizing backbone amide groups of HPr, and does not impact on the phospho-His189 of EIN. The dynamics occurs with the k(ex) of ~500 s(-1) which compares to the phosphoryl transfer rate of ~850 s(-1) between EIN and HPr. The conformational dynamics in HPr may be important for its phosphotransfer reactions with multiple partner proteins.

Masking of pressure overload in a patient with pulmonary thromboembolism accompanied by atrial septal aneurysm.

A pulmonary thromboembolism (PTE) causes a dramatic pressure overload to the right heart. Previous case reports have shown that elevated right atrial pressure secondary to a PTE can cause right-to-left shunting in the presence of an atrial septal aneurysm (ASA). A 57-year-old female with diabetes, hypertension, and an old cerebral infarction was admitted to our hospital with acute PTE. Initial transthoracic echocardiography (TTE) showed an ASA swing from the right side to the left side, and right-to-left shunting was detected immediately in the agitated saline test. However, definite signs of pressure overload of the right heart were not detected in the TTE. This educational case shows that right-to-left shunting via a patent foramen ovale in the ASA can cause normal right atrial pressure, thus masking the pressure overload of the right heart in a patient with PTE.

Prognostic value of serial global longitudinal strain measured by two-dimensional speckle tracking echocardiography in patients with ST-segment elevation myocardial infarction.

The aim of this study was to determine whether assessment of global longitudinal strain (GLS) before revascularization could predict adverse cardiac events after ST-segment elevation myocardial infarction (STEMI). In addition, the relation between GLS and cardiac biomarkers was investigated. From July 2006 through December 2009, 98 patients with first STEMI underwent conventional and speckle tracking echocardiography at initial presentation and 3 days after primary coronary intervention. Patients were divided into 3 groups according to percent changes of GLS compared to baseline GLS values: group 1, improved GLS >10%; group 2, unchanged GLS from -10% to 10%; and group 3, decreased GLS <-10%. Subsequent complications including all-cause mortality and readmission because of congestive heart failure during a 6-month period of follow-up were prospectively evaluated. After coronary intervention, GLS was improved in 29 patients (30%, group 1), unchanged in 55 patients (56%, group 2), and worsened in 14 patients (14%, group 3). Complications developed in 7 patients (group 1, n = 0, 0%; group 2, n = 2, 28%; group 3, n = 5, 72%, p <0.01). Multivariate Cox analysis showed an independent association of GLS before and after coronary intervention with subsequent complications. Significant correlations were observed between GLS and cardiac biomarkers. In conclusion, GLS assessment before coronary intervention was a good predictor of complications in patients with STEMI comparable to predictions using GLS after intervention at 6-month follow-up.