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Influenza viruses antagonize key immune defence mechanisms via the virulence factor non-structural protein 1 (NS1). A key mechanism of virulence by NS1 is blocking nuclear export of host messenger RNAs, including those encoding immune factors; however, the direct cellular target of NS1 and the mechanism of host mRNA export inhibition are not known. Here, we identify the target of NS1 as the mRNA export receptor complex, nuclear RNA export factor 1-nuclear transport factor 2-related export protein 1 (NXF1-NXT1), which is the principal receptor mediating docking and translocation of mRNAs through the nuclear pore complex via interactions with nucleoporins. We determined the crystal structure of NS1 in complex with NXF1-NXT1 at 3.8 Å resolution. The structure reveals that NS1 prevents binding of NXF1-NXT1 to nucleoporins, thereby inhibiting mRNA export through the nuclear pore complex into the cytoplasm for translation. We demonstrate that a mutant influenza virus deficient in binding NXF1-NXT1 does not block host mRNA export and is attenuated. This attenuation is marked by the release of mRNAs encoding immune factors from the nucleus. In sum, our study uncovers the molecular basis of a major nuclear function of influenza NS1 protein that causes potent blockage of host gene expression and contributes to inhibition of host immunity.
Multiprotein machines drive virtually all primary cellular processes. Modular multidomain proteins are widely distributed within these dynamic complexes because they provide the flexibility needed to remodel structure as well as rapidly assemble and disassemble components of the machinery. Understanding the functional dynamics of modular multidomain proteins is a major challenge confronting structural biology today because their structure is not fixed in time. Small-angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) spectroscopy have proven particularly useful for the analysis of the structural dynamics of modular multidomain proteins because they provide highly complementary information for characterizing the architectural landscape accessible to these proteins. SAXS provides a global snapshot of all architectural space sampled by a molecule in solution. Furthermore, SAXS is sensitive to conformational changes, organization and oligomeric states of protein assemblies, and the existence of flexibility between globular domains in multiprotein complexes. The power of NMR to characterize dynamics provides uniquely complementary information to the global snapshot of the architectural ensemble provided by SAXS because it can directly measure domain motion. In particular, NMR parameters can be used to define the diffusion of domains within modular multidomain proteins, connecting the amplitude of interdomain motion to the architectural ensemble derived from SAXS. Our laboratory has been studying the roles of modular multidomain proteins involved in human DNA replication using SAXS and NMR. Here, we present the procedure for acquiring and analyzing SAXS and NMR data, using DNA primase and replication protein A as examples.
© 2017 Elsevier Inc. All rights reserved.
While much is known about the molecular pathways that regulate embryonic development and adult homeostasis of the endocrine pancreas, little is known about what regulates early postnatal development and maturation of islets. Given that birth marks the first exposure to enteral nutrition, we investigated how nutrient-regulated signaling pathways influence postnatal islet development in mice. We performed loss-of-function studies of mechanistic target of rapamycin (mTOR), a highly conserved kinase within a nutrient-sensing pathway known to regulate cellular growth, morphogenesis and metabolism. Deletion of Mtor in pancreatic endocrine cells had no significant effect on their embryonic development. However, within the first 2 weeks after birth, mTOR-deficient islets became dysmorphic, β-cell maturation and function were impaired, and animals lost islet mass. Moreover, we discovered that these distinct functions of mTOR are mediated by separate downstream branches of the pathway, in that mTORC1 (with adaptor protein Raptor) is the main complex mediating the maturation and function of islets, whereas mTORC2 (with adaptor protein Rictor) impacts islet mass and architecture. Taken together, these findings suggest that nutrient sensing may be an essential trigger for postnatal β-cell maturation and islet development.
© 2017. Published by The Company of Biologists Ltd.
Unconventional myosin 7a (Myo7a), myosin 7b (Myo7b), and myosin 15a (Myo15a) all contain MyTH4-FERM domains (myosin tail homology 4-band 4.1, ezrin, radixin, moesin; MF) in their cargo binding tails and are essential for the growth and function of microvilli and stereocilia. Numerous mutations have been identified in the MyTH4-FERM tandems of these myosins in patients suffering visual and hearing impairment. Although a number of MF domain binding partners have been identified, the molecular basis of interactions with the C-terminal MF domain (CMF) of these myosins remains poorly understood. Here we report the high-resolution crystal structure of Myo7b CMF in complex with the extended PDZ3 domain of USH1C (a.k.a., Harmonin), revealing a previously uncharacterized interaction mode both for MyTH4-FERM tandems and for PDZ domains. We predicted, based on the structure of the Myo7b CMF/USH1C PDZ3 complex, and verified that Myo7a CMF also binds to USH1C PDZ3 using a similar mode. The structure of the Myo7b CMF/USH1C PDZ complex provides mechanistic explanations for >20 deafness-causing mutations in Myo7a CMF. Taken together, these findings suggest that binding to PDZ domains, such as those from USH1C, PDZD7, and Whirlin, is a common property of CMFs of Myo7a, Myo7b, and Myo15a.
Proteasomal degradation of topoisomerase I (topoI) is one of the most remarkable cellular phenomena observed in response to camptothecin (CPT). Importantly, the rate of topoI degradation is linked to CPT resistance. Formation of the topoI-DNA-CPT cleavable complex inhibits DNA re-ligation resulting in DNA-double strand break (DSB). The degradation of topoI marks the first step in the ubiquitin proteasome pathway (UPP) dependent DNA damage response (DDR). Here, we show that the Ku70/Ku80 heterodimer binds with topoI, and that the DNA-dependent protein kinase (DNA-PKcs) phosphorylates topoI on serine 10 (topoI-pS10), which is subsequently ubiquitinated by BRCA1. A higher basal level of topoI-pS10 ensures rapid topoI degradation leading to CPT resistance. Importantly, PTEN regulates DNA-PKcs kinase activity in this pathway and PTEN deletion ensures DNA-PKcs dependent higher topoI-pS10, rapid topoI degradation and CPT resistance.
The mammalian target of rapamycin (mTOR) is a crucial signaling node that integrates environmental cues to regulate cell survival, proliferation and metabolism, and is often deregulated in human cancer. mTOR kinase acts in two functionally distinct complexes, mTOR complex 1 (mTORC1) and 2 (mTORC2), whose activities and substrate specificities are regulated by complex co-factors. Deregulation of this centralized signaling pathway has been associated with a variety of human diseases including diabetes, neurodegeneration and cancer. Although mTORC1 signaling has been extensively studied in cancer, recent discoveries indicate a subset of human cancers harboring amplifications in mTORC2-specific genes as the only actionable genomic alterations, suggesting a distinct role for mTORC2 in cancer as well. This review will summarize recent advances in dissecting the relative contributions of mTORC1 versus mTORC2 in cancer, their role in tumor-associated blood vessels and tumor immunity, and provide an update on mTOR inhibitors.
Macrophage apoptosis and the ability of macrophages to clean up dead cells, a process called efferocytosis, are crucial determinants of atherosclerosis lesion progression and plaque stability. Environmental stressors initiate endoplasmic reticulum (ER) stress and activate the unfolded protein response (UPR). Unresolved ER stress with activation of the UPR initiates apoptosis. Macrophages are resistant to apoptotic stimuli, because of activity of the PI3K/Akt pathway. Macrophages express 3 Akt isoforms, Akt1, Akt2 and Akt3, which are products of distinct but homologous genes. Akt displays isoform-specific effects on atherogenesis, which vary with different vascular cell types. Loss of macrophage Akt2 promotes the anti-inflammatory M2 phenotype and reduces atherosclerosis. However, Akt isoforms are redundant with regard to apoptosis. c-Jun NH-terminal kinase (JNK) is a pro-apoptotic effector of the UPR, and the JNK1 isoform opposes anti-apoptotic Akt signaling. Loss of JNK1 in hematopoietic cells protects macrophages from apoptosis and accelerates early atherosclerosis. IκB kinase α (IKKα, a member of the serine/threonine protein kinase family) plays an important role in mTORC2-mediated Akt signaling in macrophages, and IKKα deficiency reduces macrophage survival and suppresses early atherosclerosis. Efferocytosis involves the interaction of receptors, bridging molecules, and apoptotic cell ligands. Scavenger receptor class B type I is a critical mediator of macrophage efferocytosis via the Src/PI3K/Rac1 pathway in atherosclerosis. Agonists that resolve inflammation offer promising therapeutic potential to promote efferocytosis and prevent atherosclerotic clinical events. (Circ J 2016; 80: 2259-2268).
The single-span membrane protein KCNE3 modulates a variety of voltage-gated ion channels in diverse biological contexts. In epithelial cells, KCNE3 regulates the function of the KCNQ1 potassium ion (K(+)) channel to enable K(+) recycling coupled to transepithelial chloride ion (Cl(-)) secretion, a physiologically critical cellular transport process in various organs and whose malfunction causes diseases, such as cystic fibrosis (CF), cholera, and pulmonary edema. Structural, computational, biochemical, and electrophysiological studies lead to an atomically explicit integrative structural model of the KCNE3-KCNQ1 complex that explains how KCNE3 induces the constitutive activation of KCNQ1 channel activity, a crucial component in K(+) recycling. Central to this mechanism are direct interactions of KCNE3 residues at both ends of its transmembrane domain with residues on the intra- and extracellular ends of the KCNQ1 voltage-sensing domain S4 helix. These interactions appear to stabilize the activated "up" state configuration of S4, a prerequisite for full opening of the KCNQ1 channel gate. In addition, the integrative structural model was used to guide electrophysiological studies that illuminate the molecular basis for how estrogen exacerbates CF lung disease in female patients, a phenomenon known as the "CF gender gap."
Germinal centres (GCs) promote humoral immunity and vaccine efficacy. In GCs, antigen-activated B cells proliferate, express high-affinity antibodies, promote antibody class switching, and yield B cell memory. Whereas the cytokine milieu has long been known to regulate effector functions that include the choice of immunoglobulin class, both cell-autonomous and extrinsic metabolic programming have emerged as modulators of T-cell-mediated immunity. Here we show in mice that GC light zones are hypoxic, and that low oxygen tension () alters B cell physiology and function. In addition to reduced proliferation and increased B cell death, low impairs antibody class switching to the pro-inflammatory IgG2c antibody isotype by limiting the expression of activation-induced cytosine deaminase (AID). Hypoxia induces HIF transcription factors by restricting the activity of prolyl hydroxyl dioxygenase enzymes, which hydroxylate HIF-1α and HIF-2α to destabilize HIF by binding the von Hippel-Landau tumour suppressor protein (pVHL). B-cell-specific depletion of pVHL leads to constitutive HIF stabilization, decreases antigen-specific GC B cells and undermines the generation of high-affinity IgG, switching to IgG2c, early memory B cells, and recall antibody responses. HIF induction can reprogram metabolic and growth factor gene expression. Sustained hypoxia or HIF induction by pVHL deficiency inhibits mTOR complex 1 (mTORC1) activity in B lymphoblasts, and mTORC1-haploinsufficient B cells have reduced clonal expansion, AID expression, and capacities to yield IgG2c and high-affinity antibodies. Thus, the normal physiology of GCs involves regional variegation of hypoxia, and HIF-dependent oxygen sensing regulates vital functions of B cells. We propose that the restriction of oxygen in lymphoid organs, which can be altered in pathophysiological states, modulates humoral immunity.
HER2 overexpression drives Akt signaling and cell survival and HER2-enriched breast tumors have a poor outcome when Akt is upregulated. Akt is activated by phosphorylation at T308 via PI3K and S473 via mTORC2. The importance of PI3K-activated Akt signaling is well documented in HER2-amplified breast cancer models, but the significance of mTORC2-activated Akt signaling in this setting remains uncertain. We report here that the mTORC2 obligate cofactor Rictor is enriched in HER2-amplified samples, correlating with increased phosphorylation at S473 on Akt. In invasive breast cancer specimens, Rictor expression was upregulated significantly compared with nonmalignant tissues. In a HER2/Neu mouse model of breast cancer, genetic ablation of Rictor decreased cell survival and phosphorylation at S473 on Akt, delaying tumor latency, penetrance, and burden. In HER2-amplified cells, exposure to an mTORC1/2 dual kinase inhibitor decreased Akt-dependent cell survival, including in cells resistant to lapatinib, where cytotoxicity could be restored. We replicated these findings by silencing Rictor in breast cancer cell lines, but not silencing the mTORC1 cofactor Raptor (RPTOR). Taken together, our findings establish that Rictor/mTORC2 signaling drives Akt-dependent tumor progression in HER2-amplified breast cancers, rationalizing clinical investigation of dual mTORC1/2 kinase inhibitors and developing mTORC2-specific inhibitors for use in this setting. Cancer Res; 76(16); 4752-64. ©2016 AACR.
©2016 American Association for Cancer Research.