Substrate-Induced Allosteric Change in the Quaternary Structure of the Spermidine N-Acetyltransferase SpeG

AbstractThe spermidine N-acetyltransferase SpeG is a dodecameric enzyme that catalyzes the transfer of an acetyl group from acetyl coenzyme A to polyamines such as spermidine and spermine. SpeG has an allosteric polyamine-binding site and acetylating polyamines regulate their intracellular concentrations. The structures of SpeG from Vibrio cholerae in complexes with polyamines and cofactor have been characterized earlier. Here, we present the dodecameric structure of SpeG from V. cholerae in a ligand-free form in three different conformational states: open, intermediate and closed. All structures were crystallized in C2 space group symmetry and contain six monomers in the asymmetric unit cell. Two hexamers related by crystallographic 2-fold symmetry form the SpeG dodecamer. The open and intermediate states have a unique open dodecameric ring. This SpeG dodecamer is asymmetric except for the one 2-fold axis and is unlike any known dodecameric structure. Using a fluorescence thermal shift assay, size-exclusion chromatography with multi-angle light scattering, small-angle X-ray scattering analysis, negative-stain electron microscopy and structural analysis, we demonstrate that this unique open dodecameric state exists in solution. Our combined results indicate that polyamines trigger conformational changes and induce the symmetric closed dodecameric state of the protein when they bind to their allosteric sites

Eastern Seaboard Electric Grid Fragility Maps Supporting Persistent Availability

Persistently available power transmission can be disrupted by weather causing power outages with economic and social consequences. This research investigated the effects on the national power grid from a specific weather event, Hurricane Irene, that caused approximately 5.7 million customer power outages along the Eastern Seaboard in August of 2011. The objective was to describe the geographic differences in the grid s vulnerability to these events. Individual factors, such as wind speed or precipitation, were correlated with the number of outages to determine the greatest mechanism of power failure in hopes of strengthening the future power grid. The resulting fragility maps not only depicted 18 counties that were less robust than the design-standard robustness model and three counties that were more robust, but also drew new damage contours with correlated wind speeds and county features

Combined SAXS/EM Based Models of the S. elongatus Post-Translational Circadian Oscillator and its Interactions with the Output His-Kinase SasA

The circadian clock in the cyanobacterium Synechococcus elongatus is composed of a post-translational oscillator (PTO) that can be reconstituted in vitro from three different proteins in the presence of ATP and a transcription-translation feedback loop (TTFL). The homo-hexameric KaiC kinase, phosphatase and ATPase alternates between hypo- and hyper-phosphorylated states over the 24-h cycle, with KaiA enhancing phosphorylation, and KaiB antagonizing KaiA and promoting KaiC subunit exchange. SasA is a His kinase that relays output signals from the PTO formed by the three Kai proteins to the TTFL. Although the crystal structures for all three Kai proteins are known, atomic resolution structures of Kai and Kai/SasA protein complexes have remained elusive. Here, we present models of the KaiAC and KaiBC complexes derived from solution small angle X-ray scattering (SAXS), which are consistent with previous EM based models. We also present a combined SAXS/EM model of the KaiC/SasA complex, which has two N-terminal SasA sensory domains occupying positions on the C-terminal KaiC ring reminiscent of the orientations adopted by KaiB dimers. Using EM we demonstrate that KaiB and SasA compete for similar binding sites on KaiC. We also propose an EM based model of the ternary KaiABC complex that is consistent with the sequestering of KaiA by KaiB on KaiC during the PTO dephosphorylation phase. This work provides the first 3D-catalogue of protein-protein interactions in the KaiABC PTO and the output pathway mediated by SasA

Solution structures of DNA-bound gyrase

The DNA gyrase negative supercoiling mechanism involves the assembly of a large gyrase/DNA complex and conformational rearrangements coupled to ATP hydrolysis. To establish the complex arrangement that directs the reaction towards negative supercoiling, bacterial gyrase complexes bound to 137- or 217-bp DNA fragments representing the starting conformational state of the catalytic cycle were characterized by sedimentation velocity and small-angle X-ray scattering (SAXS) experiments. The experiments revealed elongated complexes with hydrodynamic radii of 70–80 Å. Molecular envelopes calculated from these SAXS data show 2-fold symmetric molecules with the C-terminal domain (CTD) of the A subunit and the ATPase domain of the B subunit at opposite ends of the complexes. The proposed gyrase model, with the DNA binding along the sides of the molecule and wrapping around the CTDs located near the exit gate of the protein, adds new information on the mechanism of DNA negative supercoiling

Conductive Network and β Polymorph Content Evolution Caused by Thermal Treatment in Carbon Nanotubes-BaTiO 3 Hybrids Reinforced Polyvinylidene Fluoride Composites

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Conductive Network and β Polymorph Content Evolution Caused by Thermal Treatment in Carbon Nanotubes-BaTiO₃ Hybrids Reinforced Polyvinylidene Fluoride Composites

A good dispersion of carbon nanotube (CNT) in polyvinylidene fluoride (PVDF) is realized by using CNT and BaTiO₃ (BT) hybrids (H-CNT-BT) with a special core–shell structure. Thus, a high dielectric performance is achieved for the composite (H-CNT-BT/PVDF). Carried by BT, CNT is easy to connect with each other and thus more interface area may be created which helps to achieve an extremely low percolation threshold (<i>f</i>_c). Moreover, the dielectric permittivity of the composite near <i>f</i>_c is increased more than three times after thermal treatment while dielectric loss remains at a low level. In order to study more comprehensively about the influence of thermal treatment, in situ synchrotron X-ray is used to detect recrystalline behavior of PVDF. Results of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) show that after thermal treatment, the content of β polymorph has increased nearly double times at the interface of CNT-PVDF, and the thickness of amorphous layers (<i>L</i>_a) in PVDF’s long periods (<i>L</i>_p) has shrunk around 10 Å. Increased β polymorph at the interface of CNT-PVDF may form an ideal structure with the grading dielectric permittivities from the center to the border which decreases the contrast between CNT and PVDF. Meanwhile, the evolution of CNT’s network possibly occurs in the procedure of <i>L</i>_a shrinkage, where the strong interfacial polarization may be aroused. Besides, an increase in the thickness of crystalline lamella may also arouse more orientational polarization and improve dielectric properties at high frequency. Combining with BT’s buffer role for blocking possible leakage current during the percolative behavior, the dielectric loss of composite can remain at a very low level even after thermal treatment. In addition, experimental results show that prolonging annealing duration or increasing annealing cycles favors stabilization of CNT’s dynamic percolation, which reduces the sensitivity of CNT’s network in the composite and further improves dielectric properties. After thermal treatment, the dielectric permittivity reachs 1172, but dielectric loss remains at 0.55 at 100 Hz. To our best knowledge, this high dielectric performance is really rare, only found in recent reports