Oxysterols direct immune cell migration via EBI2

Epstein-Barr virus-induced gene 2 (EBI2, also known as GPR183) is a G-protein-coupled receptor that is required for humoral immune responses; polymorphisms in the receptor have been associated with inflammatory autoimmune diseases. The natural ligand for EBI2 has been unknown. Here we describe the identification of 7α,25-dihydroxycholesterol (also called 7α,25-OHC or 5-cholesten-3β,7α,25-triol) as a potent and selective agonist of EBI2. Functional activation of human EBI2 by 7α,25-OHC and closely related oxysterols was verified by monitoring second messenger readouts and saturable, high-affinity radioligand binding. Furthermore, we find that 7α,25-OHC and closely related oxysterols act as chemoattractants for immune cells expressing EBI2 by directing cell migration in vitro and in vivo. A critical enzyme required for the generation of 7α,25-OHC is cholesterol 25-hydroxylase (CH25H). Similar to EBI2 receptor knockout mice, mice deficient in CH25H fail to position activated B cells within the spleen to the outer follicle and mount a reduced plasma cell response after an immune challenge. This demonstrates that CH25H generates EBI2 biological activity in vivo and indicates that the EBI2-oxysterol signalling pathway has an important role in the adaptive immune response.     

Jamming by shear

A broad class of disordered materials including foams, glassy molecular systems, colloids and granular materials can form jammed states. A jammed system can resist small stresses without deforming irreversibly, whereas unjammed systems flow under any applied stresses. The broad applicability of the Liu-Nagel jamming concept has attracted intensive theoretical and modelling interest but has prompted less experimental effort. In the Liu-Nagel framework, jammed states of athermal systems exist only above a certain critical density. Although numerical simulations for particles that do not experience friction broadly support this idea, the nature of the jamming transition for frictional grains is less clear. Here we show that jamming of frictional, disk-shaped grains can be induced by the application of shear stress at densities lower than the critical value, at which isotropic (shear-free) jamming occurs. These jammed states have a much richer phenomenology than the isotropic jammed states: for small applied shear stresses, the states are fragile, with a strong force network that percolates only in one direction. A minimum shear stress is needed to create robust, shear-jammed states with a strong force network percolating in all directions. The transitions from unjammed to fragile states and from fragile to shear-jammed states are controlled by the fraction of force-bearing grains. The fractions at which these transitions occur are statistically independent of the density. Jammed states with densities lower than the critical value have an anisotropic fabric (contact network). The minimum anisotropy of shear-jammed states vanishes as the density approaches the critical value from below, in a manner reminiscent of an order-disorder transition.     

Structures of APC/C(Cdh1) with substrates identify Cdh1 and Apc10 as the D-box co-receptor

The ubiquitylation of cell-cycle regulatory proteins by the large multimeric anaphase-promoting complex (APC/C) controls sister chromatid segregation and the exit from mitosis. Selection of APC/C targets is achieved through recognition of destruction motifs, predominantly the destruction (D)-box and KEN (Lys-Glu-Asn)-box. Although this process is known to involve a co-activator protein (either Cdc20 or Cdh1) together with core APC/C subunits, the structural basis for substrate recognition and ubiquitylation is not understood. Here we investigate budding yeast APC/C using single-particle electron microscopy and determine a cryo-electron microscopy map of APC/C in complex with the Cdh1 co-activator protein (APC/C(Cdh1)) bound to a D-box peptide at ～10 ? resolution. We find that a combined catalytic and substrate-recognition module is located within the central cavity of the APC/C assembled from Cdh1, Apc10--a core APC/C subunit previously implicated in substrate recognition--and the cullin domain of Apc2. Cdh1 and Apc10, identified from difference maps, create a co-receptor for the D-box following repositioning of Cdh1 towards Apc10. Using NMR spectroscopy we demonstrate specific D-box-Apc10 interactions, consistent with a role for Apc10 in directly contributing towards D-box recognition by the APC/C(Cdh1) complex. Our results rationalize the contribution of both co-activator and core APC/C subunits to D-box recognition and provide a structural framework for understanding mechanisms of substrate recognition and catalysis by the APC/C.