Atomic structure of Li_2-xMnO₃ studied by neutron diffraction and STEM

Li2MnO3 is an important cathode material with extra high capacity (>300mAh g-1 for the first charge process). The exact charge-discharge mechanism and the structure evolution still remain controversial. Here the atomic structures of Li2MnO3 after partial delithiation and lithiation are investigated by neutron powder diffraction and spherical aberration-corrected scanning transmission electron microscopy (STEM). Neutron diffraction experiments are performed on Li2-xMnO3 (x=0, 0.25) in a bulk level. It can be found that the volume of the unit cell almost keeps constant, while the lattice constants in the a, b direction increases after the chemical delithiation, but the c direction decreases. For the delithiated compound Li1.75MnO3, the Li occupancies are 0.7(+-0.3), 0.9(+-0.1) for the 2c and 4h sites, respectively, resulting in the Li-concentration of 1.75(+-0.27), while the 2b sites are fully occupied. Furhtermore, the isotropic thermal vibration factors of the 2c and 4h Li atoms are considerably larger than that of the 2b Li atoms, also seemingly implying the feasible delithiation of Li atoms at the 2c and 4h sites in the Li-O layer. It is interesting to note that the thermal factor of Mn atoms is slightly larger than O atoms, which probably means that the Mn atoms are more mobile than O atoms. The STEM results suggest that the Li ions can be extracted both from the LiMn2 planes and Li planes, and the Mn ions can move reversibly in the (110) plane during delithiation and lithiation.

Structural basis for filament formation of MAVS on mitochondria by cryoEM

Mitochondrial anti-viral signaling (MAVS) protein is required for innate immune responses against RNA viruses. In virus-infected cells MAVS forms prion-like aggregates to activate antiviral signaling cascades, but the underlying structural mechanism is unknown. Here we report cryo-electron microscopic structures of the helical filaments formed by both the N-terminal caspase activation and recruitment domain (CARD) of MAVS and a truncated MAVS lacking part of the proline-rich region and the C-terminal transmembrane domain. Iterative helical real space refinement was used to analyze cryoEM images of the filaments. The CARD filament structure was resolved at 9.6 angstrom with rod-like densities fitting with four alpha helices of the domain. That of the truncated MAVS was resolved at 16.4 angstroms, showing the arrangement of the middle segment of MAVS at the periphery of the CARD filament. Both structures are left-handed three-stranded helical filaments, revealing specific interfaces between individual CARD subunits that are dictated by electrostatic interactions between neighboring strands and hydrophobic interactions within each strand. Point mutations at multiple locations of these two interfaces impaired filament formation and antiviral signaling. Super-resolution imaging of virus-infected cells revealed rod-shaped MAVS clusters on mitochondria. These results elucidate the structural mechanism of MAVS polymerization, and explain how an α-helical domain uses distinct chemical interactions to form self-perpetuating filaments.

Structural basis of DNA sequence recognition by the response regulator PhoP

The PhoP-PhoR two-component system plays a key role in regulating virulence of Mycobacterium tuberculosis. The response regulator PhoP is a transcription regulator, and it regulates expression of more than 100 genes. PhoP belongs to the OmpR/PhoB subfamily of response regulators. Despite extensive research in recent years, the molecular mechanism of DNA sequence recognition by this large subfamily of response regulators is not fully understood, especially the role of the N-terminal regulatory domain on DNA binding of the effector domain. Here we present a crystal structure of the full-length PhoP in complex with a direct-repeat DNA sequence. PhoP binds to DNA as a dimer. The two effector domains bind in tandem, each interacting with a half site of the direct repeat DNA. The DNA recognition helix inserts into the major groove, reading the sequence of a 7-bp motif. The wing residues interact with the downstream sequence, with a conserved arginine side chain inserting into the minor groove. Surprisingly, the regulatory domain also forms a tandem arrangement, instead of the anticipated symmetric dimer. The regulatory domain of the upstream protomer interacts with both domains of the downstream protomer. The structural elements of the alpha4-beta5-alpha5 face, which often found to be involved in dimer interface of the regulatory domain, play important roles in the interactions between the protomers. The crystal structure explains why PhoP recognizes direct repeats of two 7-bp motifs with a strict spacing of 4 bp and the highly cooperative binding of the two monomers. Detailed analysis of the structure along with analysis of the DNA sequence requirements and ITC measurements of protein-DNA binding interactions will be presented.