Amino acid ligation to cognate transfer RNAs (tRNAs) is catalyzed by aminoacyl-tRNA synthetases (aaRSs)-essential interpreters of the genetic code during translation. Mammalian cells harbor 20 ...cytoplasmic aaRSs, out of which 9 (in 8 proteins), with 3 non-aaRS proteins, AIMPs 1 to 3, form the ∼1.25-MDa multi-tRNA synthetase complex (MSC). The function of MSC remains uncertain, as does its mechanism of assembly. Constituents of multiprotein complexes encounter obstacles during assembly, including inappropriate interactions, topological constraints, premature degradation of unassembled subunits, and suboptimal stoichiometry. To facilitate orderly and efficient complex formation, some complexes are assembled cotranslationally by a mechanism in which a fully formed, mature protein binds a nascent partner as it emerges from the translating ribosome. Here, we show out of the 121 possible interaction events between the 11 MSC constituents, 15 are cotranslational. AIMPs are involved in the majority of these cotranslational interactions, suggesting they are not only critical for MSC structure but also for assembly. Unexpectedly, several cotranslational events involve more than the usual dyad of interacting proteins. We show two modes of cotranslational interaction, namely a "multisite" mechanism in which two or more mature proteins bind the same nascent peptide at distinct sites and a second "piggy-back" mechanism in which a mature protein carries a second fully formed protein and binds to a single site on an emerging peptide. Multimodal mechanisms of cotranslational interaction offer a diversity of pathways for ordered, piecewise assembly of small subcomplexes into larger heteromultimeric complexes such as the mammalian MSC.
ABSTRACT
In the developing nervous system, neural stem cells (NSCs) use temporal patterning to generate a wide variety of different neuronal subtypes. In Drosophila, the temporal transcription ...factors, Hunchback, Kruppel, Pdm and Castor, are sequentially expressed by NSCs to regulate temporal identity during neurogenesis. Here, we identify a new temporal transcription factor that regulates the transition from the Pdm to Castor temporal windows. This factor, which we call Chronophage (or ‘time-eater’), is homologous to mammalian CTIP1 (Bcl11a) and CTIP2 (Bcl11b). We show that Chronophage binds upstream of the castor gene and regulates its expression. Consistent with Chronophage promoting a temporal switch, chronophage mutants generate an excess of Pdm-specified neurons and are delayed in generating neurons associated with the Castor temporal window. In addition to promoting the Pdm to Castor transition, Chronophage also represses the production of neurons generated during the earlier Hunchback and Kruppel temporal windows. Genetic interactions with Hunchback and Kruppel indicate that Chronophage regulates NSC competence to generate Hunchback- and Kruppel-specified neurons. Taken together, our results suggest that Chronophage has a conserved role in temporal patterning and neuronal subtype specification.
Translational control provides numerous advantages in regulation of gene expression including rapid responsiveness, intracellular localization, nondestruction of template mRNA, and coordinated ...regulation of transcript ensembles. Transcript-selective, translational control is driven by the specific interaction of factor(s) with the 5' or 3' untranslated region (UTR), thereby influencing initiation, elongation, or termination of mRNA translation. The mean length of human 3'UTRs is greater than that of 5'UTR, indicating the expanded potential for motifs, structural elements, and binding sites for trans-acting factors that exert transcript-selective translation control. New and unexpected mechanisms of 3'UTR-mediated translational control and their contributions to disease have received increasing attention during the last decade. Here, we briefly review a few recent and representative discoveries of 3'UTR-mediated translational control, emphasizing the novel aspects of these regulatory mechanisms and their potential pathophysiological significance.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Expression of vascular endothelial growth factor‐A (VEGFA) by tumour‐associated macrophages is critical for tumour progression and metastasis. Hypoxia, a common feature of the neoplastic ...microenvironment, induces VEGFA expression by increased transcription, translation, and mRNA stabilization. Here, we report a new mechanism of VEGFA regulation by hypoxia that involves reversal of microRNA (miRNA)‐mediated silencing of VEGFA expression. We show that the CA‐rich element (CARE) in the human VEGFA 3′‐UTR is targeted by at least four miRNAs. Among these miRNAs, miR‐297 and ‐299 are endogenously expressed in monocytic cells and negatively regulate VEGFA expression. Unexpectedly, hypoxia completely reverses miRNA‐mediated repression of VEGFA expression. We show that heterogeneous nuclear ribonucleoprotein L (hnRNP L), which also binds the VEGFA 3′‐UTR CARE, prevents miRNA silencing activity. Hypoxia induces translocation of nuclear hnRNP L to the cytoplasm, which markedly increases hnRNP L binding to VEGFA mRNA thereby inhibiting miRNA activity. In summary, we describe a novel regulatory mechanism in which the interplay between miRNAs and RNA‐binding proteins influences expression of a critical hypoxia‐inducible angiogenic protein. These studies may contribute to provide miRNA‐based anticancer therapeutic tools.
There is increasing evidence of interplay between miRNAs and RNA‐binding proteins in regulating protein expression. Here, the hypoxia‐induced relocalization of heterogeneous nuclear ribonucleoprotein L (hnRNP L) relieves miRNA‐mediated repression of the VEGFA mRNA by binding to the 3′‐UTR region that is targeted by several miRNAs.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Repeated domains in proteins that have undergone duplication or loss, and sequence divergence, are especially informative about phylogenetic relationships. We have exploited divergent repeats of the ...highly structured, 50-amino acid WHEP domains that join the catalytic subunits of bifunctional glutamyl-prolyl tRNA synthetase (EPRS) as a sequence-informed repeat (SIR) to trace the origin and evolution of EPRS in holozoa. EPRS is the only fused tRNA synthetase, with two distinct aminoacylation activities, and a non-canonical translation regulatory function mediated by the WHEP domains in the linker. Investigating the duplications, deletions and divergence of WHEP domains, we traced the bifunctional EPRS to choanozoans and identified the fusion event leading to its origin at the divergence of ichthyosporea and emergence of filozoa nearly a billion years ago. Distribution of WHEP domains from a single species in two or more distinct clades suggested common descent, allowing the identification of linking organisms. The discrete assortment of choanoflagellate WHEP domains with choanozoan domains as well as with those in metazoans supported the phylogenetic position of choanoflagellates as the closest sister group to metazoans. Analysis of clustering and assortment of WHEP domains provided unexpected insights into phylogenetic relationships amongst holozoan taxa. Furthermore, observed gaps in the transition between WHEP domain groupings in distant taxa allowed the prediction of undiscovered or extinct evolutionary intermediates. Analysis based on SIR domains can provide a phylogenetic counterpart to palaentological approaches of discovering "missing links" in the tree of life.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Shifting the balance away from tumor‐mediated immune suppression toward tumor immune rejection is the conceptual foundation for a variety of immunotherapy efforts currently being tested. These ...efforts largely focus on activating antitumor immune responses but are confounded by multiple immune cell populations, including myeloid‐derived suppressor cells (MDSCs), which serve to suppress immune system function. We have identified immune‐suppressive MDSCs in the brains of GBM patients and found that they were in close proximity to self‐renewing cancer stem cells (CSCs). MDSCs were selectively depleted using 5‐flurouracil (5‐FU) in a low‐dose administration paradigm, which resulted in prolonged survival in a syngeneic mouse model of glioma. In coculture studies, patient‐derived CSCs but not nonstem tumor cells selectively drove MDSC‐mediated immune suppression. A cytokine screen revealed that CSCs secreted multiple factors that promoted this activity, including macrophage migration inhibitory factor (MIF), which was produced at high levels by CSCs. Addition of MIF increased production of the immune‐suppressive enzyme arginase‐1 in MDSCs in a CXCR2‐dependent manner, whereas blocking MIF reduced arginase‐1 production. Similarly to 5‐FU, targeting tumor‐derived MIF conferred a survival advantage to tumor‐bearing animals and increased the cytotoxic T cell response within the tumor. Importantly, tumor cell proliferation, survival, and self‐renewal were not impacted by MIF reduction, demonstrating that MIF is primarily an indirect promoter of GBM progression, working to suppress immune rejection by activating and protecting immune suppressive MDSCs within the GBM tumor microenvironment. Stem Cells 2016;34:2026–2039
CSCs secrete MIF, which in turn drives MDSC‐mediated immune suppression. Otvos and colleagues report that myeloid‐derived suppressor cells (MDSCs) infiltrate the glioblastoma microenvironment and localize adjacent to treatment‐refractory cancer stem cells (CSCs). These authors demonstrate that CSCs secrete macrophage migration inhibitory factor (MIF), which is bound by C‐X‐C motif chemokine receptor 2 (CXCR2) expressed by the adjacent MDSCs. MIF binding triggers the production of arginase 1 (ARG1), which suppresses the function of cytotoxic, CD8+ T cells. In total, these investigators identify a novel immune‐suppressive CSC/MDSC interaction that negates the tumor‐rejecting effects of other resident immune cell populations and enables glioblastoma progression. They go on to demonstrate how direct targeting of the MDSC population bolsters immune activation within the tumor microenvironment and confers a survival advantage in a preclinical model of glioma.
Translational control provides numerous advantages in regulation of gene expression including rapid responsiveness, intracellular localization, nondestruction of template mRNA, and coordinated ...regulation of transcript ensembles. Transcript-selective, translational control is driven by the specific interaction of factor(s) with the 5′ or 3′ untranslated region (UTR), thereby influencing initiation, elongation, or termination of mRNA translation. The mean length of human 3′UTRs is greater than that of 5′UTR, indicating the expanded potential for motifs, structural elements, and binding sites for trans -acting factors that exert transcript-selective translation control. New and unexpected mechanisms of 3′UTR-mediated translational control and their contributions to disease have received increasing attention during the last decade. Here, we briefly review a few recent and representative discoveries of 3′UTR-mediated translational control, emphasizing the novel aspects of these regulatory mechanisms and their potential pathophysiological significance.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
LINE-1 (L1) retrotransposons can mobilize (retrotranspose) within the human genome, and mutagenic de novo L1 insertions can lead to human diseases, including cancers. As a result, cells are actively ...engaged in preventing L1 retrotransposition. This work reveals that the human Condensin II complex restricts L1 retrotransposition in both non-transformed and transformed cell lines through inhibition of L1 transcription and translation. Condensin II subunits, CAP-D3 and CAP-H2, interact with members of the Gamma-Interferon Activated Inhibitor of Translation (GAIT) complex including the glutamyl-prolyl-tRNA synthetase (EPRS), the ribosomal protein L13a, Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and NS1 associated protein 1 (NSAP1). GAIT has been shown to inhibit translation of mRNAs encoding inflammatory proteins in myeloid cells by preventing the binding of the translation initiation complex, in response to Interferon gamma (IFN-γ). Excitingly, our data show that Condensin II promotes complexation of GAIT subunits. Furthermore, RNA-Immunoprecipitation experiments in epithelial cells demonstrate that Condensin II and GAIT subunits associate with L1 RNA in a co-dependent manner, independent of IFN-γ. These findings suggest that cooperation between the Condensin II and GAIT complexes may facilitate a novel mechanism of L1 repression, thus contributing to the maintenance of genome stability in somatic cells.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Multiple eukaryotic ribosomal proteins (RPs) are co-opted for extraribosomal “moonlighting” activities, but paradoxically, RPs exhibit rapid turnover when not ribosome-bound. In one illustrative case ...of a functional extraribosomal RP, interferon (IFN)-γ induces ribosome release of L13a and assembly into the IFN-gamma-activated inhibitor of translation (GAIT) complex for translational control of a subset of inflammation-related proteins. Here we show GAPDH functions as a chaperone, shielding newly released L13a from proteasomal degradation. However, GAPDH protective activity is lost following cell treatment with oxidatively modified low density lipoprotein and IFN-γ. These agonists stimulate S-nitrosylation at Cys247 of GAPDH, which fails to interact with L13a, causing proteasomal degradation of essentially the entire cell complement of L13a and defective translational control. Evolution of extraribosomal RP activities might require coevolution of protective chaperones, and pathological disruption of either protein, or their interaction, presents an alternative mechanism of diseases due to RP defects, and targets for therapeutic intervention.
Display omitted
► GAPDH acts as a shield, protecting free ribosomal protein L13a from degradation ► Condition-dependent degradation of an extraribosomal ribosomal protein ► S-Nitrosylation of GAPDH at Cys247 suppresses its protective activity ► Degradation of unprotected L13a inactivates GAIT system and induces VEGF-A expression
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Transforming growth factor-β (TGF-β) induces epithelial-mesenchymal transdifferentiation (EMT) accompanied by cellular differentiation and migration. Despite extensive transcriptomic profiling, the ...identification of TGF-β-inducible, EMT-specific genes has met with limited success. Here we identify a post-transcriptional pathway by which TGF-β modulates the expression of EMT-specific proteins and of EMT itself. We show that heterogeneous nuclear ribonucleoprotein E1 (hnRNP E1) binds a structural, 33-nucleotide TGF-β-activated translation (BAT) element in the 3′ untranslated region of disabled-2 (Dab2) and interleukin-like EMT inducer (ILEI) transcripts, and represses their translation. TGF-β activation leads to phosphorylation at Ser 43 of hnRNP E1 by protein kinase Bβ/Akt2, inducing its release from the BAT element and translational activation of Dab2 and ILEI messenger RNAs. Modulation of hnRNP E1 expression or its post-translational modification alters the TGF-β-mediated reversal of translational silencing of the target transcripts and EMT. These results suggest the existence of a TGF-β-inducible post-transcriptional regulon that controls EMT during the development and metastatic progression of tumours.
Full text
Available for:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
PDF
