The structure of a major surface antigen SAG19 from Eimeria tenella unifies the Eimeria SAG family

In infections by apicomplexan parasites including Plasmodium, Toxoplasma gondii, and Eimeria, host interactions are mediated by proteins including families of membrane-anchored cysteine-rich surface antigens (SAGs) and SAG-related sequences (SRS). Eimeria tenella causes caecal coccidiosis in chickens and has a SAG family with over 80 members making up 1% of the proteome. We have solved the structure of a representative E. tenella SAG, EtSAG19, revealing that, despite a low level of sequence similarity, the entire Eimeria SAG family is unified by its three-layer αβα fold which is related to that of the CAP superfamily. Furthermore, sequence comparisons show that the Eimeria SAG fold is conserved in surface antigens of the human coccidial parasite Cyclospora cayetanensis but this fold is unrelated to that of the SAGs/SRS proteins expressed in other apicomplexans including Plasmodium species and the cyst-forming coccidia Toxoplasma gondii, Neospora caninum and Besnoitia besnoiti. However, despite having very different structures, Consurf analysis showed that Eimeria SAG and Toxoplasma SRS families each exhibit marked hotspots of sequence hypervariability that map to their surfaces distal to the membrane anchor. This suggests that the primary and convergent purpose of the different structures is to provide a platform onto which sequence variability can be imposed

Modeling the effects of a light bridge on properties of magnetohydrodynamic waves in solar pores

Solar pores are ideal magnetic structures for wave propagation and transport of energy radially outwards across the upper layers of the solar atmosphere. We aim to model the excitation and propagation of magnetohydrodynamic waves in a pore with a light bridge modeled as two interacting magnetic flux tubes separated by a thin, weaker-field layer. We solve the three-dimensional magnetohydrodynamic equations numerically and calculate the circulation as a measure of net torsional motion. We find that the interaction between flux tubes results in the natural excitation of propagating torsional Alfvén waves but find no torsional waves in the model with a single flux tube. The torsional Alfvén waves propagate with wave speeds matching the local Alfvén speed where wave amplitude peaks

Slow body magnetohydrodynamic waves in solar photospheric flux tubes with density inhomogeneity

Pores and sunspots are ideal environments for the propagation of guided magnetohydrodynamic (MHD) waves. However, modelling such photospheric waveguides with varying background quantities such as plasma density and magnetic field has thus far been very limited. Such modelling is required to correctly interpret MHD waves observed in pores and sunspots with resolved inhomogeneities such as light bridges and umbral dots. This study will investigate the propagation characteristics and the spatial structure of slow body MHD modes in a magnetic flux tube with a circular cross-section with inhomogeneous equilibrium density distribution under solar photospheric conditions in the short wavelength limit. For simplicity, the equilibrium density profile is taken to have a circular density enhancement or depletion. The advantage of this is that the strength, size, and position of the density inhomogeneity can be easily changed. Calculating the eigenfrequencies and eigenfunctions of the slow body modes is addressed numerically with use of the Fourier–Chebyshev Spectral method. The radial and azimuthal variation of eigenfunctions is obtained by solving a Helmholtz-type partial differential equation with Dirichlet boundary conditions. The inhomogeneous equilibrium density profile results in modified eigenvalues and eigenvectors. It was found that a localized density inhomogeneity leads to a decrease in the eigenvalues and the spatial structure of modes ceases to be a global harmonic oscillation, as the modes migrate towards regions of lower density. Comparing the homogeneous case and the cases corresponding to depleted density enhancement, the dimensionless phase speed undergoes a significant drop in its value (at least 40 per cent)

Purification, crystallization and quaternary structure analysis of a glycerol dehydrogenase S305C mutant from Bacillus stearothermophilus

Bacillus stearothermophilus glycerol dehydrogenase (GlyDH) is a 39.5 kDa molecular weight metalloenzyme which catalyzes the oxidation of glycerol to dihydroxyacetone with the concomitant reduction of NAD+ to NADH. Despite its classification as a member of the 'iron-containing' polyol dehydrogenase family, studies on recombinant B. stearothermophilus GlyDH have shown this enzyme to be Zn2+-dependent. Crystals of a S305C GlyDH mutant were obtained by the hanging-drop vapour-diffusion method, using ammonium sulfate and PEG 400 as precipitating agents, in the presence and absence of NAD+. The crystals belong to space group I422, with approximate unit-cell parameters a = b = 105, c = 149 Å and one subunit in the asymmetric unit, corresponding to a packing density of 2.6 Å3 Da-1. The crystals diffract X-rays to at least 1.8 Å resolution on a synchrotron-radiation source. Determination of the structure will provide insights into the key determinations of catalytic activity of this class of enzymes, for which no structures are currently available