Fast inhibitory neurotransmission in the mammalian nervous system is largely mediated by GABAA receptors, chloride-selective members of the superfamily of pentameric Cys-loop receptors. Native GABAA receptors are heteromeric assemblies sensitive to many important drugs, from sedatives to anesthetics and anticonvulsant agents, with mutant forms of GABAA receptors implicated in multiple neurological diseases. Despite the profound importance of heteromeric GABAA receptors in neuroscience and medicine, they have proven recalcitrant to structure determination. Here we present the structure of a tri-heteromeric α1β1γ2SEM GABAA receptor in complex with GABA, determined by single particle cryo-EM at 3.1–3.8 Å resolution, elucidating molecular principles of receptor assembly and agonist binding. Remarkable N-linked glycosylation on the α1 subunit occludes the extracellular vestibule of the ion channel and is poised to modulate receptor assembly and perhaps ion channel gating. Our work provides a pathway to structural studies of heteromeric GABAA receptors and a framework for rational design of novel therapeutic agents.

Members of the NrdH family of redox proteins, which consists of small glutaredoxin-like proteins with thioredoxin-like activity, serve as the reducing partners of class Ib ribonucleotide reductases. Here, we report the crystal structure of NrdH from Mycobacterium tuberculosis, refined to a crystallographic R factor of 14.02% (Rfree = 15.53%) at 0.87 Å resolution. The tertiary structure of M. tuberculosis NrdH has a typical thioredoxin fold as expected. The extremely high resolution of the structure allows us to dissect the functionality of the protein in great depth. Structural superimposition of M. tuberculosis NrdH and thioredoxin reductase over the Escherichia coli thioredoxin reductase-thioredoxin complex suggests the ability of NrdH to accept electrons from M. tuberculosis thioredoxin reductase. This raises the important question of why glutaredoxins are unable to accept electrons from thioredoxin reductases and why thioredoxins are unable to reduce ribonucleotide reductases. Furthermore, forms of NrdH from other organisms have been shown to be a specific reductant of class Ib ribonucleotide reductases. We attempt to explain this substrate specificity by modeling the C-terminal peptide of a ribunucleotide subunit, NrdE, in the active site of NrdH using the already available Grx-NrdA-Cter-peptide structure. Statistical coupling analysis of NrdH, glutaredoxins, and thioredoxins reveals different sets of co-evolving contiguous clusters of amino acid residues, which might explain the differences in the biochemical properties of these structurally similar yet functionally distinct subclasses of proteins.

The transcription factor Rv0081 of Mycobacterium tuberculosis controls hypoxic gene expression and acts as a regulatory hub in the latent phase of tuberculosis (TB) infection. We report here the crystal structure of Rv0081 at 2.9 Å resolution revealing that it belongs to the well‐known ArsR/SmtB family proteins. However, unlike other members in this family, Rv0081 has neither a metal‐binding domain nor does it possess Cys residues, suggesting an alternate mechanism of gene regulation. Our structural and biochemical analyses suggest the molecular basis for the recognition of self‐regulatory DNA sequences and a plausible mechanism of regulation of Rv0081 in the latent phase of TB infection. Database Structural data are available in the Protein Data Bank under the accession number – http://www.rcsb.org/pdb/search/structidSearch.do?structureId=6JMI