Structural basis of the Integrator complex assembly and association with transcription factors.

Integrator is a multi-subunit protein complex responsible for premature transcription termination of coding and non-coding RNAs. This is achieved via two enzymatic activities, RNA endonuclease and protein phosphatase, acting on the promoter-proximally paused RNA polymerase Ⅱ (RNAPⅡ). Yet, it remains unclear how Integrator assembly and recruitment are regulated and what the functions of many of its core subunits are. Here, we report the structures of two human Integrator sub-complexes: INTS10/13/14/15 and INTS5/8/10/15, and an integrative model of the fully assembled Integrator bound to the RNAPⅡ paused elongating complex (PEC). An in silico protein-protein interaction screen of over 1,500 human transcription factors (TFs) identified ZNF655 as a direct interacting partner of INTS13 within the fully assembled Integrator. We propose a model wherein INTS13 acts as a platform for the recruitment of TFs that could modulate the stability of the Integrator's association at specific loci and regulate transcription attenuation of the target genes.

Supramolecular assembly of the Escherichia coli LdcI upon acid stress.

Pathogenic and commensal bacteria often have to resist the harsh acidity of the host stomach. The inducible lysine decarboxylase LdcI buffers the cytosol and the local extracellular environment to ensure enterobacterial survival at low pH. Here, we investigate the acid stress-response regulation of Escherichia coli LdcI by combining biochemical and biophysical characterization with negative stain and cryoelectron microscopy (cryo-EM) and wide-field and superresolution fluorescence imaging. Due to deleterious effects of fluorescent protein fusions on native LdcI decamers, we opt for three-dimensional localization of nanobody-labeled endogenous wild-type LdcI in acid-stressed E. coli cells and show that it organizes into distinct patches at the cell periphery. Consistent with recent hypotheses that in vivo clustering of metabolic enzymes often reflects their polymerization as a means of stimulus-induced regulation, we show that LdcI assembles into filaments in vitro at physiologically relevant low pH. We solve the structures of these filaments and of the LdcI decamer formed at neutral pH by cryo-EM and reveal the molecular determinants of LdcI polymerization, confirmed by mutational analysis. Finally, we propose a model for LdcI function inside the enterobacterial cell, providing a structural and mechanistic basis for further investigation of the role of its supramolecular organization in the acid stress response.

Characterization of thrombogenic, endothelial and inflammatory markers in supraventricular tachycardia: a study in patients with structurally normal hearts.

Patients with atrial fibrillation (AF) are at an increased risk of thromboembolism and stroke primarily from the development of thrombi within the left atrium. Pathological changes in blood constituents and atrial endothelial damage promote left atrial thrombus formation. It is not known whether factors predisposing to left atrial thrombus formation in AF are disease specific or also evident within the normal heart. The present study examined whether there are differences in platelet reactivity, endothelial function and inflammation in blood samples obtained from intracardiac and peripheral sites in subjects within structurally normal hearts. Sixteen patients with diagnosed left-sided supraventricular tachycardia (SVT) undergoing a routine elective electrophysiological study and ablation were investigated. Blood samples were taken simultaneously from the femoral vein, right atrium and left atrium, immediately following trans-septal puncture and prior to heparin bolus administration. Between peripheral and atrial sample sites, patients with SVT showed no change in platelet reactivity or aggregation (P-selectin (CD62P) P = 0.91; platelet-derived soluble CD40 ligand P = 0.9), thrombus formation (thrombin-antithrombin complex; P = 0.55), endothelial function (von Willebrand factor P = 0.75; asymmetric dimethylarginine (ADMA) P = 0.97; nitric oxide P = 0.61), or inflammation (vascular cell adhesion molecule-1 P = 0.59; intercellular adhesion molecule-1 (ICAM-1) P = 0.69). However, SVT patients had lower ADMA and ICAM-1 levels than AF patients. The present study demonstrates, for the first time, that SVT subjects with structurally normal hearts have consistent haemostatic function between atrial and peripheral sites. These results suggest that the atria of SVT patients do not contain predisposing thrombogenic, endothelial or inflammatory factors that promote and/or initiate thrombus formation.

Structure of the human 20S U5 snRNP.

The 20S U5 small nuclear ribonucleoprotein particle (snRNP) is a 17-subunit RNA-protein complex and a precursor of the U4/U6.U5 tri-snRNP, the major building block of the precatalytic spliceosome. CD2BP2 is a hallmark protein of the 20S U5 snRNP, absent from the mature tri-snRNP. Here we report a high-resolution cryogenic electron microscopy structure of the 20S U5 snRNP, shedding light on the mutually exclusive interfaces utilized during tri-snRNP assembly and the role of the CD2BP2 in facilitating this process.

A combinatorial approach to uncover an additional Integrator subunit.

RNA polymerase II (RNAPII) controls expression of all protein-coding genes and most noncoding loci in higher eukaryotes. Calibrating RNAPII activity requires an assortment of polymerase-associated factors that are recruited at sites of active transcription. The Integrator complex is one of the most elusive transcriptional regulators in metazoans, deemed to be recruited after initiation to help establish and modulate paused RNAPII. Integrator is known to be composed of 14 subunits that assemble and operate in a modular fashion. We employed proteomics and machine-learning structure prediction (AlphaFold2) to identify an additional Integrator subunit, INTS15. We report that INTS15 assembles primarily with the INTS13/14/10 module and interfaces with the Int-PP2A module. Functional genomics analysis further reveals a role for INTS15 in modulating RNAPII pausing at a subset of genes. Our study shows that omics approaches combined with AlphaFold2-based predictions provide additional insights into the molecular architecture of large and dynamic multiprotein complexes.

Structural and biochemical characterisation of the Providencia stuartii arginine decarboxylase shows distinct polymerisation and regulation.

Bacterial homologous lysine and arginine decarboxylases play major roles in the acid stress response, physiology, antibiotic resistance and virulence. The Escherichia coli enzymes are considered as their archetypes. Whereas acid stress triggers polymerisation of the E. coli lysine decarboxylase LdcI, such behaviour has not been observed for the arginine decarboxylase Adc. Here we show that the Adc from a multidrug-resistant human pathogen Providencia stuartii massively polymerises into filaments whose cryo-EM structure reveals pronounced differences between Adc and LdcI assembly mechanisms. While the structural determinants of Adc polymerisation are conserved only in certain Providencia and Burkholderia species, acid stress-induced polymerisation of LdcI appears general for enterobacteria. Analysis of the expression, activity and oligomerisation of the P. stuartii Adc further highlights the distinct properties of this unusual protein and lays a platform for future investigation of the role of supramolecular assembly in the superfamily or arginine and lysine decarboxylases.

The AAA+ ATPase RavA and its binding partner ViaA modulate E. coli aminoglycoside sensitivity through interaction with the inner membrane.

Enteric bacteria have to adapt to environmental stresses in the human gastrointestinal tract such as acid and nutrient stress, oxygen limitation and exposure to antibiotics. Membrane lipid composition has recently emerged as a key factor for stress adaptation. The E. coli ravA-viaA operon is essential for aminoglycoside bactericidal activity under anaerobiosis but its mechanism of action is unclear. Here we characterise the VWA domain-protein ViaA and its interaction with the AAA+ ATPase RavA, and find that both proteins localise at the inner cell membrane. We demonstrate that RavA and ViaA target specific phospholipids and subsequently identify their lipid-binding sites. We further show that mutations abolishing interaction with lipids restore induced changes in cell membrane morphology and lipid composition. Finally we reveal that these mutations render E. coli gentamicin-resistant under fumarate respiration conditions. Our work thus uncovers a ravA-viaA-based pathway which is mobilised in response to aminoglycosides under anaerobiosis and engaged in cell membrane regulation.

Structural and functional analysis of the Francisella lysine decarboxylase as a key actor in oxidative stress resistance.

Francisella tularensis is one of the most virulent pathogenic bacteria causing the acute human respiratory disease tularemia. While the mechanisms underlying F. tularensis pathogenesis are largely unknown, previous studies have shown that a F. novicida transposon mutant with insertions in a gene coding for a putative lysine decarboxylase was attenuated in mouse spleen, suggesting a possible role of its protein product as a virulence factor. Therefore, we set out to structurally and functionally characterize the F. novicida lysine decarboxylase, which we termed LdcF. Here, we investigate the genetic environment of ldcF as well as its evolutionary relationships with other basic AAT-fold amino acid decarboxylase superfamily members, known as key actors in bacterial adaptative stress response and polyamine biosynthesis. We determine the crystal structure of LdcF and compare it with the most thoroughly studied lysine decarboxylase, E. coli LdcI. We analyze the influence of ldcF deletion on bacterial growth under different stress conditions in dedicated growth media, as well as in infected macrophages, and demonstrate its involvement in oxidative stress resistance. Finally, our mass spectrometry-based quantitative proteomic analysis enables identification of 80 proteins with expression levels significantly affected by ldcF deletion, including several DNA repair proteins potentially involved in the diminished capacity of the F. novicida mutant to deal with oxidative stress. Taken together, we uncover an important role of LdcF in F. novicida survival in host cells through participation in oxidative stress response, thereby singling out this previously uncharacterized protein as a potential drug target.

Structural insights into ATP hydrolysis by the MoxR ATPase RavA and the LdcI-RavA cage-like complex.

The hexameric MoxR AAA+ ATPase RavA and the decameric lysine decarboxylase LdcI form a 3.3 MDa cage, proposed to assist assembly of specific respiratory complexes in E. coli. Here, we show that inside the LdcI-RavA cage, RavA hexamers adopt an asymmetric spiral conformation in which the nucleotide-free seam is constrained to two opposite orientations. Cryo-EM reconstructions of free RavA reveal two co-existing structural states: an asymmetric spiral, and a flat C2-symmetric closed ring characterised by two nucleotide-free seams. The closed ring RavA state bears close structural similarity to the pseudo two-fold symmetric crystal structure of the AAA+ unfoldase ClpX, suggesting a common ATPase mechanism. Based on these structures, and in light of the current knowledge regarding AAA+ ATPases, we propose different scenarios for the ATP hydrolysis cycle of free RavA and the LdcI-RavA cage-like complex, and extend the comparison to other AAA+ ATPases of clade 7.