Cryo-EM Structures of Centromeric Tri-nucleosomes Containing a Central CENP-A Nucleosome

The histone H3 variant CENP-A is a crucial epigenetic marker for centromere specification. CENP-A forms a characteristic nucleosome and dictates the higher-order configuration of centromeric chromatin. However, little is known about how the CENP-A nucleosome affects the architecture of centromeric chromatin. In this study, we reconstituted tri-nucleosomes mimicking a centromeric nucleosome arrangement containing the CENP-A nucleosome, and determined their 3D structures by cryoelectron microscopy. The H3-CENP-A-H3 tri-nucleosomes adopt an untwisted architecture, with an outward-facing linker DNA path between nucleosomes. This is distinct from the H3-H3-H3 tri-nucleosome architecture, with an inward-facing DNA path. Intriguingly, the untwisted architecture may allow the CENP-A nucleosome to be exposed to the solvent in the condensed chromatin model. These results provide a structural basis for understanding the 3D configuration of CENP-A-containing chromatin, and may explain how centromeric proteins can specifically target the CENP-A nucleosomes buried in robust amounts of H3 nucleosomes in centromeres

Silica phase as a thermoluminescence phosphor in ALH-77214 (L3.4)chondrite

The induced thermoluminescence (TL) images for a slice sample of ALH-77214 (L3.4) have been measured to examine TL phosphors in type 3 ordinary chondrites with low TL sensitivity, using a TL spatial distribution readout system combined with microscope. The chemical compositions of TL phosphors were analyzed by X-ray microanalyzer. The TL emissions due to silica phases have been newly observed in two pyroxene chondrules with silica inclusions (SiO_2=97-99wt%) and one porphyritic olivine chondrule with very silica-rich mesostasis (SiO_2=84wt%), in addition to those due to feldspar crystals formed in chondrule mesostases enriched in plagioclase component. The glow curves of the silica phases (the peaks around 240-280℃) are quite different from those of the usual chondrule mesostases (the peaks around 80-120℃). We have tried to utilize glow curves of induced TL for five silica phases (hydrothermal quartz, volcanic quartz, tridymite, cristobalite and silica glass). From similarities of the shape of glow curve, the silica phase was tentatively identified to be cristobalite. Utilizing silica phases as a common TL phosphor, we can make a comparative study of TL characteristics among chondritic meteorites

Assembly mechanism of the pleomorphic immature poxvirus scaffold

In Vaccinia virus (VACV), the prototype poxvirus, scaffold protein D13 forms a honeycomb-like lattice on the viral membrane that results in formation of the pleomorphic immature virion (IV). The structure of D13 is similar to those of major capsid proteins that readily form icosahedral capsids in nucleocytoplasmic large DNA viruses (NCLDVs). However, the detailed assembly mechanism of the nonicosahedral poxvirus scaffold has never been understood. Here we show the cryo-EM structures of the D13 trimer and scaffold intermediates produced in vitro. The structures reveal that the displacement of the short N-terminal α-helix is critical for initiation of D13 self-assembly. The continuous curvature of the IV is mediated by electrostatic interactions that induce torsion between trimers. The assembly mechanism explains the semiordered capsid-like arrangement of D13 that is distinct from icosahedral NCLDVs. Our structures explain how a single protein can self-assemble into different capsid morphologies and represent a local exception to the universal Caspar-Klug theory of quasi-equivalence

Structure of the bacterial flagellar hook cap provides insights into a hook assembly mechanism

Assembly of bacterial flagellar hook requires FlgD, a protein known to form the hook cap. Symmetry mismatch between the hook and the hook cap is believed to drive efficient assembly of the hook in a way similar to the filament cap helping filament assembly. However, the hook cap dependent mechanism of hook assembly has remained poorly understood. Here, we report the crystal structure of the hook cap composed of five subunits of FlgD from Salmonella enterica at 3.3 Å resolution. The pentameric structure of the hook cap is divided into two parts: a stalk region composed of five N-terminal domains; and a petal region containing five C-terminal domains. Biochemical and genetic analyses show that the N-terminal domains of the hook cap is essential for the hook-capping function, and the structure now clearly reveals why. A plausible hook assembly mechanism promoted by the hook cap is proposed based on the structure