Although iron is required to sustain life, its free concentration and metabolism have to be tightly regulated. This is achieved through a variety of iron-binding proteins including transferrin and ferritin. During infection, bacteria acquire much of their iron from the host by synthesizing siderophores that scavenge iron and transport it into the pathogen. We recently demonstrated that enterochelin, a bacterial catecholate siderophore, binds to the host protein lipocalin 2 (ref. 5). Here, we show that this event is pivotal in the innate immune response to bacterial infection. Upon encountering invading bacteria the Toll-like receptors on immune cells stimulate the transcription, translation and secretion of lipocalin 2; secreted lipocalin 2 then limits bacterial growth by sequestrating the iron-laden siderophore. Our finding represents a new component of the innate immune system and the acute phase response to infection.

Protein mechanostability is a fundamental biological property that can only be measured by single-molecule manipulation techniques. Such studies have unveiled a variety of highly mechanostable modules (mainly of the Ig-like, ␤-sandwich type) in modular proteins subjected to mechanical stress from the cytoskeleton and the metazoan cell-cell interface. Their mechanostability is often attributed to a ''mechanical clamp'' of secondary structure (a patch of backbone hydrogen bonds) fastening their ends. Here we investigate the nanomechanics of scaffoldins, an important family of scaffolding proteins that assembles a variety of cellulases into the so-called cellulosome, a microbial extracellular nanomachine for cellulose adhesion and degradation. These proteins anchor the microbial cell to cellulose substrates, which makes their connecting region likely to be subjected to mechanical stress. By using singlemolecule force spectroscopy based on atomic force microscopy, polyprotein engineering, and computer simulations, here we show that the cohesin I modules from the connecting region of cellulosome scaffoldins are the most robust mechanical proteins studied experimentally or predicted from the entire Protein Data Bank. The mechanostability of the cohesin modules studied correlates well with their mechanical kinetic stability but not with their thermal stability, and it is well predicted by computer simulations, even coarse-grained. This extraordinary mechanical stability is attributed to 2 mechanical clamps in tandem. Our findings provide the current upper limit of protein mechanostability and establish shear mechanical clamps as a general structural/functional motif widespread in proteins putatively subjected to mechanical stress. These data have important implications for the scaffoldin physiology and for protein design in biotechnology and nanotechnology.cellulosome ͉ cohesin ͉ mechanical stability ͉ protein nanomechanics ͉ single-molecule force spectroscopy

SUMMARY The function of G protein-coupled receptors (GPCRs) can be modulated by a number of endogenous allosteric molecules. In this study, we used molecular dynamics, radioligand binding and thermostability experiments to elucidate the role of the recently discovered sodium ion binding site in the allosteric modulation of the human A2A adenosine receptor, conserved among class A GPCRs. While the binding of antagonists and sodium ions to the receptor was non-competitive in nature, the binding of agonists and sodium ions appears to require mutually exclusive conformational states of the receptor. Amiloride analogs can also bind to the sodium binding pocket showing distinct patterns of agonist and antagonist modulation. These findings suggest that physiological concentrations of sodium ions affect functionally relevant conformational states of GPCRs, and can help to design novel synthetic allosteric modulators or bitopic ligands exploiting the sodium ion binding pocket.

The transcription factor ATF3 inhibits lipid body formation in macrophages during atherosclerosis in part by dampening the expression of cholesterol 25-hydroxylase.

Inflammasomes are cytosolic protein complexes that regulate caspase-1 activation and the secretion of interleukin-1␤ (IL-1␤) and IL-18. Several different inflammasome complexes have been identified, but the NLRP3 inflammasome is particularly notable because of its central role in diseases of inflammation. Recent work has demonstrated an essential role for the NLRP3 inflammasome in host defense against influenza virus. We show here that two other RNA viruses, encephalomyocarditis virus (EMCV) and vesicular stomatitis virus (VSV), activate the NLRP3 inflammasome in dendritic cells and macrophages through a mechanism requiring viral replication. Inflammasome activation in response to both viruses does not require MDA5 or RIG-I signaling. Despite the ability of the NLRP3 inflammasome to detect EMCV and VSV, wild-type and caspase-1-deficient mice were equally susceptible to infection with both viruses. These findings indicate that the NLRP3 inflammasome may be a common pathway for RNA virus detection, but its precise role in the host response may be variable.Interleukin-1␤ (IL-1␤) and IL-18 are two pleiotropic cytokines that play crucial roles in inflammatory responses in addition to instructing adaptive immune responses. Secretion of both is controlled at two steps: transcription and posttranslational processing. Therefore, any pathogen that elicits IL-1␤/ IL-18 secretion must be able to provide both signals. Expression of both pro-IL-1␤ and pro-IL-18 is induced by NF-B (for example via Toll-like receptor [TLR] activation) although pro-IL-18 is expressed at a basal level. Posttranslational processing and secretion of IL-1␤ and IL-18 are controlled by caspase-1, whose activity is regulated by a cytosolic protein complex known as the inflammasome (4).Most inflammasomes consist of a member of the Nod-like receptor (NLR) family of cytosolic receptors that either directly interacts with caspase-1 or is indirectly coupled to it by the adaptor protein ASC (apoptsis-associated speck-like protein containing a CARD). When an agonist is detected by an NLR, the NLR oligomerizes and recruits caspase-1, which is proteolytically processed to its active form. Activated caspase-1 in turn processes IL-1␤ and IL-18, resulting in the secretion of the mature cytokines (14).Several inflammasomes have now been described. The NLRP3 inflammasome is unique in one respect: unlike other inflammasomes, which detect discrete agonists, the NLRP3 inflammasome is activated by a broad array of agonists, including extracellular ATP, intracellular nucleic acids, crystals, bacteria, and fungi. How these agonists induce NLRP3 activation is not clear although several hypotheses have been proposed (13,22,26).Recent work suggests that the inflammasome may play a role in antiviral host defense. Both vaccinia virus and murine cytomegalovirus are detected by the AIM2 inflammasome, which responds to cytosolic double-stranded DNA (18), while influenza A virus is detected by the NLRP3 inflammasome (1, 9, 21). In this work, we show that the RNA viruses encephalo...