Cryo-EM structure of a dimeric B-Raf:14-3-3 complex reveals asymmetry in the active sites of B-Raf kinases

Raf kinases are important cancer drug targets. Paradoxically, many B-Raf inhibitors induce the activation of Raf kinases. Cryo-electron microscopy structural analysis of a phosphorylated B-Raf kinase domain dimer in complex with dimeric 14-3-3, at a resolution of ~3.9 angstroms, shows an asymmetric arrangement in which one kinase is in a canonical "active" conformation. The distal segment of the C-terminal tail of this kinase interacts with, and blocks, the active site of the cognate kinase in this asymmetric arrangement. Deletion of the C-terminal segment reduces Raf activity. The unexpected asymmetric quaternary architecture illustrates how the paradoxical activation of Raf by kinase inhibitors reflects an innate mechanism, with 14-3-3 facilitating inhibition of one kinase while maintaining activity of the other. Conformational modulation of these contacts may provide new opportunities for Raf inhibitor development.

Frontiers in Cryo Electron Microscopy of Complex Macromolecular Assemblies

In recent years, cryo electron microscopy (cryo-EM) technology has been transformed with the development of better instrumentation, direct electron detectors, improved methods for specimen preparation, and improved software for data analysis. Analyses using single-particle cryo-EM methods have enabled determination of structures of proteins with sizes smaller than 100 kDa and resolutions of ∼2 Å in some cases. The use of electron tomography combined with subvolume averaging is beginning to allow the visualization of macromolecular complexes in their native environment in unprecedented detail. As a result of these advances, solutions to many intractable challenges in structural and cell biology, such as analysis of highly dynamic soluble and membrane-embedded protein complexes or partially ordered protein aggregates, are now within reach. Recent reports of structural studies of G protein-coupled receptors, spliceosomes, and fibrillar specimens illustrate the progress that has been made using cryo-EM methods, and are the main focus of this review.

Human aminoacyl-tRNA synthetases in diseases of the nervous system

Aminoacyl-tRNA synthetases (AaRSs) are ubiquitously expressed enzymes that ensure accurate translation of the genetic information into functional proteins. These enzymes also execute a variety of non-canonical functions that are significant for regulation of diverse cellular processes and that reside outside the realm of protein synthesis. Associations between faults in AaRS-mediated processes and human diseases have been long recognized. Most recent research findings strongly argue that 10 cytosolic and 14 mitochondrial AaRSs are implicated in some form of pathology of the human nervous system. The advent of modern whole-exome sequencing makes it all but certain that similar associations between the remaining 15 ARS genes and neurologic illnesses will be defined in future. It is not surprising that an intense scientific debate about the role of translational machinery, in general, and AaRSs, in particular, in the development and maintenance of the healthy human neural cell types and the brain is sparked. Herein, we summarize the current knowledge about causative links between mutations in human AaRSs and diseases of the nervous system and briefly discuss future directions.

The crystal structure of human GlnRS provides basis for the development of neurological disorders

Cytosolic glutaminyl-tRNA synthetase (GlnRS) is the singular enzyme responsible for translation of glutamine codons. Compound heterozygous mutations in GlnRS cause severe brain disorders by a poorly understood mechanism. Herein, we present crystal structures of the wild type and two pathological mutants of human GlnRS, which reveal, for the first time, the domain organization of the intact enzyme and the structure of the functionally important N-terminal domain (NTD). Pathological mutations mapping in the NTD alter the domain structure, and decrease catalytic activity and stability of GlnRS, whereas missense mutations in the catalytic domain induce misfolding of the enzyme. Our results suggest that the reduced catalytic efficiency and a propensity of GlnRS mutants to misfold trigger the disease development. This report broadens the spectrum of brain pathologies elicited by protein misfolding and provides a paradigm for understanding the role of mutations in aminoacyl-tRNA synthetases in neurological diseases.

Interactions of epigallo-catechin 3-gallate and ovalbumin, the major allergen of egg white

Polyphenols, the potent plant secondary metabolites, have beneficial effects on human health, but the mechanism(s) by which these effects are exerted is not well understood. Here, we present the detailed analysis of the interactions between the major green tea catechin, epigallo-catechin 3-gallate (EGCG), and the major dietary protein and allergen, ovalbumin (OVA). We show that EGCG binds to the pocket that partly overlaps with the previously identified IgE-binding region in OVA, and that this interaction induces structural changes in the allergen. Moreover, our ex vivo studies reveal that OVA binds IgE and stimulates degranulation of basophils, and that its uptake by monocytes proceeds at a slower rate in the presence of EGCG. This study provides further evidence in support of the proposed mechanism by which EGCG interactions with the food allergens contribute to its diverse biological activities and may impair antigen uptake by antigen-presenting cells.