The cytoplasmic capping complex assembles on adapter protein nck1 bound to the proline-rich C-terminus of Mammalian capping enzyme

Adapting to change: resolving the dynamic and dual roles of NCK1 and NCK2

Adaptor proteins play central roles in the assembly of molecular complexes and co-ordinated activation of specific pathways. Through their modular domain structure, the NCK family of adaptor proteins (NCK1 and NCK2) link protein targets via their single SRC Homology (SH) 2 and three SH3 domains. Classically, their SH2 domain binds to phosphotyrosine motif-containing receptors (e.g. receptor tyrosine kinases), while their SH3 domains bind polyproline motif-containing cytoplasmic effectors. Due to these functions being established for both NCK1 and NCK2, their roles were inaccurately assumed to be redundant. However, in contrast with this previously held view, NCK1 and NCK2 now have a growing list of paralog-specific functions, which underscores the need to further explore their differences. Here we review current evidence detailing how these two paralogs are unique, including differences in their gene/protein regulation, binding partners and overall contributions to cellular functions. To help explain these contrasting characteristics, we then discuss SH2/SH3 structural features, disordered interdomain linker regions and post-translational modifications. Together, this review seeks to highlight the importance of distinguishing NCK1 and NCK2 in research and to pave the way for investigations into the origins of their interaction specificity.

Trypanosome mRNA recapping is triggered by hypermethylation originating from cap 4

RNA methylation adjacent to the 5' cap plays a critical role in controlling mRNA stability and protein synthesis. In trypanosomes the 5'-terminus of mRNA is protected by hypermethylated cap 4. Trypanosomes encode a cytoplasmic recapping enzyme TbCe1 which possesses an RNA kinase and guanylyltransferase activities that can convert decapped 5'-monophosphate-terminated pRNA into GpppRNA. Here, we demonstrated that the RNA kinase activity is stimulated by two orders of magnitude on a hypermethylated pRNA derived from cap 4. The N6, N6-2'-O trimethyladenosine modification on the first nucleotide was primarily accountable for enhancing both the RNA kinase and the guanylyltransferase activity of TbCe1. In contrast, N6 methyladenosine severely inhibits the guanylyltransferase activity of the mammalian capping enzyme. Furthermore, we showed that TbCmt1 cap (guanine N7) methyltransferase was localized in the cytoplasm, and its activity was also stimulated by hypermethylation at 2'-O ribose, suggesting that TbCe1 and TbCmt1 act together as a recapping enzyme to regenerate translatable mRNA from decapped mRNA. Our result establishes the functional role of cap 4 hypermethylation in recruitment and activation of mRNA recapping pathway. Methylation status at the 5'-end of transcripts could serve as a chemical landmark to selectively regulate the level of functional mRNA by recapping enzymes.

The mRNA-capping enzyme localizes to stress granules in the cytoplasm and maintains cap homeostasis of target mRNAs

The model of RNA stability has undergone a transformative shift with the revelation of a cytoplasmic capping activity that means a subset of transcripts are recapped autonomously of their nuclear counterparts. The present study demonstrates nucleo-cytoplasmic shuttling of the mRNA-capping enzyme (CE, also known as RNA guanylyltransferase and 5'-phosphatase; RNGTT), traditionally acknowledged for its nuclear localization and functions, elucidating its contribution to cytoplasmic capping activities. A unique nuclear export sequence in CE mediates XPO1-dependent nuclear export of CE. Notably, during sodium arsenite-induced oxidative stress, cytoplasmic CE (cCE) congregates within stress granules (SGs). Through an integrated approach involving molecular docking and subsequent co-immunoprecipitation, we identify eIF3b, a constituent of SGs, as an interactive associate of CE, implying that it has a potential role in guiding cCE to SGs. We measured the cap status of specific mRNA transcripts from U2OS cells that were non-stressed, stressed and recovered from stress, which indicated that cCE-target transcripts lost their caps during stress but remarkably regained cap stability during the recovery phase. This comprehensive study thus uncovers a novel facet of cytoplasmic CE, which facilitates cellular recovery from stress by maintaining cap homeostasis of target mRNAs.

METTL3 alters capping enzyme expression and its activity on ribosomal proteins

The 5' cap, catalyzed by RNA guanylyltransferase and 5'-phosphatase (RNGTT), is a vital mRNA modification for the functionality of mRNAs. mRNA capping occurs in the nucleus for the maturation of the functional mRNA and in the cytoplasm for fine-tuning gene expression. Given the fundamental importance of RNGTT in mRNA maturation and expression there is a need to further investigate the regulation of RNGTT. N6-methyladenosine (m 6 A) is one of the most abundant RNA modifications involved in the regulation of protein translation, mRNA stability, splicing, and export. We sought to investigate whether m 6 A could regulate the expression and activity of RNGTT. A motif for the m 6 A writer methyltransferase 3 (METTL3) in the 3'UTR of RNGTT mRNA was identified. Knockdown of METTL3 resulted in destabilizing RNGTT mRNA, and reduced protein expression. Sequencing of capped mRNAs identified an underrepresentation of ribosomal protein mRNA overlapping with 5' terminal oligopyrimidine (TOP) mRNAs and genes are dysregulated when cytoplasmic capping is inhibited. Pathway analysis identified disruptions in the mTOR and p70S6K pathways. A reduction in RPS6 mRNA capping, protein expression, and phosphorylation was detected with METTL3 knockdown.

Regulation and function of poised mRNAs in lymphocytes

Pre-existing but untranslated or 'poised' mRNA exists as a means to rapidly induce the production of specific proteins in response to stimuli and as a safeguard to limit the actions of these proteins. The translation of poised mRNA enables immune cells to express quickly genes that enhance immune responses. The molecular mechanisms that repress the translation of poised mRNA and, upon stimulation, enable translation have yet to be elucidated. They likely reflect intrinsic properties of the mRNAs and their interactions with trans-acting factors that direct poised mRNAs away from or into the ribosome. Here, I discuss mechanisms by which this might be regulated.

Long noncoding RNAs are substrates for cytoplasmic capping enzyme

Cytoplasmic capping returns a cap to specific mRNAs, thus protecting uncapped RNAs from decay. Prior to the identification of cytoplasmic capping, uncapped mRNAs were thought to be degraded. Here, we test whether long noncoding RNAs (lncRNAs) are substrates of the cytoplasmic capping enzyme (cCE). The subcellular localisation of 14 lncRNAs associated with sarcomas were examined in U2OS osteosarcoma cells. We used 5' rapid amplification of cDNA ends (RACE) to assay uncapped forms of these lncRNAs. Inhibiting cytoplasmic capping elevated uncapped forms of selected lncRNAs indicating a plausible role of cCE in targeting them. Analysis of published cap analysis of gene expression (CAGE) data shows increased prevalence of certain 5'-RACE cloned sequences, suggesting that these uncapped lncRNAs are targets of cytoplasmic capping.

Nck adaptors at a glance

The non-catalytic region of tyrosine kinase (Nck) family of adaptors, consisting of Nck1 and Nck2, contributes to selectivity and specificity in the flow of cellular information by recruiting components of signaling networks. Known to play key roles in cytoskeletal remodeling, Nck adaptors modulate host cell-pathogen interactions, immune cell receptor activation, cell adhesion and motility, and intercellular junctions in kidney podocytes. Genetic inactivation of both members of the Nck family results in embryonic lethality; however, viability of mice lacking either one of these adaptors suggests partial functional redundancy. In this Cell Science at a Glance and the accompanying poster, we highlight the molecular organization and functions of the Nck family, focusing on key interactions and pathways, regulation of cellular processes, development, homeostasis and pathogenesis, as well as emerging and non-redundant functions of Nck1 compared to those of Nck2. This article thus aims to provide a timely perspective on the biology of Nck adaptors and their potential as therapeutic targets.

A novel RNA pol II CTD interaction site on the mRNA capping enzyme is essential for its allosteric activation

Recruitment of the mRNA capping enzyme (CE/RNGTT) to the site of transcription is essential for the formation of the 5' mRNA cap, which in turn ensures efficient transcription, splicing, polyadenylation, nuclear export and translation of mRNA in eukaryotic cells. The CE GTase is recruited and activated by the Serine-5 phosphorylated carboxyl-terminal domain (CTD) of RNA polymerase II. Through the use of molecular dynamics simulations and enhanced sampling techniques, we provide a systematic and detailed characterization of the human CE-CTD interface, describing the effect of the CTD phosphorylation state, length and orientation on this interaction. Our computational analyses identify novel CTD interaction sites on the human CE GTase surface and quantify their relative contributions to CTD binding. We also identify, for the first time, allosteric connections between the CE GTase active site and the CTD binding sites, allowing us to propose a mechanism for allosteric activation. Through binding and activity assays we validate the novel CTD binding sites and show that the CDS2 site is essential for CE GTase activity stimulation. Comparison of the novel sites with cocrystal structures of the CE-CTD complex in different eukaryotic taxa reveals that this interface is considerably more conserved than previous structures have indicated.

NCK1-AS1 promotes the progression of lung squamous cell carcinoma through transcriptionally upregulating NCK1 via interacting with MYC

Lung squamous cell carcinoma (LUSC) is a prevalent subtype of nonsmall cell lung cancer (NSCLC). Dysregulated long noncoding RNAs (lncRNAs) are increasingly identified as pivotal modulators in cancer progression. NCK1 divergent transcript (NCK1-AS1) is a lncRNA that has been proven to be oncogenic in different types of human cancers. However, whether it exerts similar functions in LUSC remains to be elusive. The present study focused on investigating the influence of NCK1-AS1 on the cellular process in LUSC and exploring its underlying mechanism. Through online bioinformatics analysis, we obtained a high NCK1-AS1 level in LUSC tissues. Meanwhile, we confirmed that NCK1-AS1 was upregulated in LUSC cells. Gain- or loss-of-function assays suggested that NCK1-AS1 prompted cell proliferation and migration, whilst impeded cell apoptosis in LUSC. Mechanistically, we revealed that NCK1-AS1 induced the upregulation of its nearby gene NCK adaptor protein 1 (NCK1) at the transcriptional level by interacting with the transcription factor MYC proto-oncogene (MYC). Rescue assays indicated that NCK1 participated in the regulation of NCK1-AS1 on LUSC progression. In conclusion, we firstly demonstrated the oncogenic role of NCK1-AS1 in LUSC and illustrated its downstream molecular mechanism.

Cytoplasmic mRNA recapping has limited impact on proteome complexity

The m7G cap marks the 5' end of all eukaryotic mRNAs, but there are also capped ends that map downstream within spliced exons. A portion of the mRNA transcriptome undergoes a cyclical process of decapping and recapping, termed cap homeostasis, which impacts the translation and stability of these mRNAs. Blocking cytoplasmic capping results in the appearance of uncapped 5' ends at native cap sites but also near downstream cap sites. If translation initiates at these sites the products would lack the expected N-terminal sequences, raising the possibility of a link between mRNA recapping and proteome complexity. We performed a shotgun proteomics analysis on cells carrying an inducible inhibitor of cytoplasmic capping. A total of 21 875 tryptic peptides corresponding to 3565 proteins were identified in induced and uninduced cells. Of these, only 29 proteins significantly increased, and 28 proteins significantly decreased, when cytoplasmic capping was inhibited, indicating mRNA recapping has little overall impact on protein expression. In addition, overall peptide coverage per protein did not change significantly when cytoplasmic capping was inhibited. Together with previous work, our findings indicate cap homeostasis functions primarily in gating mRNAs between translating and non-translating states, and not as a source of proteome complexity.

Inhibition of cytoplasmic cap methylation identifies 5' TOP mRNAs as recapping targets and reveals recapping sites downstream of native 5' ends

Cap homeostasis is the cyclical process of decapping and recapping that maintains the translation and stability of a subset of the transcriptome. Previous work showed levels of some recapping targets decline following transient expression of an inactive form of RNMT (ΔN-RNMT), likely due to degradation of mRNAs with improperly methylated caps. The current study examined transcriptome-wide changes following inhibition of cytoplasmic cap methylation. This identified mRNAs with 5′-terminal oligopyrimidine (TOP) sequences as the largest single class of recapping targets. Cap end mapping of several TOP mRNAs identified recapping events at native 5′ ends and downstream of the TOP sequence of EIF3K and EIF3D. This provides the first direct evidence for downstream recapping. Inhibition of cytoplasmic cap methylation was also associated with mRNA abundance increases for a number of transcription, splicing, and 3′ processing factors. Previous work suggested a role for alternative polyadenylation in target selection, but this proved not to be the case. However, inhibition of cytoplasmic cap methylation resulted in a shift of upstream polyadenylation sites to annotated 3′ ends. Together, these results solidify cap homeostasis as a fundamental process of gene expression control and show cytoplasmic recapping can impact regulatory elements present at the ends of mRNA molecules.

A Benzophenone-Based Photocaging Strategy for the N7 Position of Guanosine

Selective modification of nucleobases with photolabile caging groups enables the study and control of processes and interactions of nucleic acids. Numerous positions on nucleobases have been targeted, but all involve formal substitution of a hydrogen atom with a photocaging group. Nature, however, also uses ring-nitrogen methylation, such as m7 G and m1 A, to change the electronic structure and properties of RNA and control biomolecular interactions essential for translation and turnover. We report that aryl ketones such as benzophenone and α-hydroxyalkyl ketone are photolabile caging groups if installed at the N7 position of guanosine or the N1 position of adenosine. Common photocaging groups derived from the ortho-nitrobenzyl moiety were not suitable. Both chemical and enzymatic methods for site-specific modification of N7G in nucleosides, dinucleotides, and RNA were developed, thereby opening the door to studying the molecular interactions of m7 G and m1 A with spatiotemporal control.

mRNA 5' ends targeted by cytoplasmic recapping cluster at CAGE tags and select transcripts are alternatively spliced

Until cytoplasmic recapping was discovered, decapping was thought to irreversibly destine an mRNA to degradation. Contradicting this idea, we readily observe mRNAs targeted by cytoplasmic capping in uncapped, yet stable forms. 5' rapid amplification of cDNA ends (RACE) shows that nearly all uncapped ends correspond to capped analysis of gene expression tags and that the recapping of ZNF207 mRNA may be restricted to a single splice isoform. Here, a modified RACE approach detected uncapped 5' RNA ends mapping to 46 mRNAs in cells expressing a dominant negative cytoplasmic capping enzyme and in normal cells. Eleven of 46 cloned mRNAs also contained splice isoform-limiting sequences. Collectively, these data reinforce earlier work and suggest that alternative splicing may play a role in targeting transcripts for - and/or determining the position of - cytoplasmic capping.

The complex enzymology of mRNA decapping: Enzymes of four classes cleave pyrophosphate bonds

The 5' ends of most RNAs are chemically modified to enable protection from nucleases. In bacteria, this is often achieved by keeping the triphosphate terminus originating from transcriptional initiation, while most eukaryotic mRNAs and small nuclear RNAs have a 5'→5' linked N7 -methyl guanosine (m7 G) cap added. Several other chemical modifications have been described at RNA 5' ends. Common to all modifications is the presence of at least one pyrophosphate bond. To enable RNA turnover, these chemical modifications at the RNA 5' end need to be reversible. Dependent on the direction of the RNA decay pathway (5'→3' or 3'→5'), some enzymes cleave the 5'→5' cap linkage of intact RNAs to initiate decay, while others act as scavengers and hydrolyse the cap element of the remnants of the 3'→5' decay pathway. In eukaryotes, there is also a cap quality control pathway. Most enzymes involved in the cleavage of the RNA 5' ends are pyrophosphohydrolases, with only a few having (additional) 5' triphosphonucleotide hydrolase activities. Despite the identity of their enzyme activities, the enzymes belong to four different enzyme classes. Nudix hydrolases decap intact RNAs as part of the 5'→3' decay pathway, DXO family members mainly degrade faulty RNAs, members of the histidine triad (HIT) family are scavenger proteins, while an ApaH-like phosphatase is the major mRNA decay enzyme of trypanosomes, whose RNAs have a unique cap structure. Many novel cap structures and decapping enzymes have only recently been discovered, indicating that we are only beginning to understand the mechanisms of RNA decapping. This article is categorized under: RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms RNA Turnover and Surveillance > Regulation of RNA Stability RNA Processing > Capping and 5' End Modifications.

Networks of mRNA Processing and Alternative Splicing Regulation in Health and Disease

mRNA processing events introduce an intricate layer of complexity into gene expression processes, supporting a tremendous level of diversification of the genome's coding and regulatory potential, particularly in vertebrate species. The recent development of massive parallel sequencing methods and their adaptation to the identification and quantification of different RNA species and the dynamics of mRNA metabolism and processing has generated an unprecedented view over the regulatory networks that are established at this level, which contribute to sustain developmental, tissue specific or disease specific gene expression programs. In this chapter, we provide an overview of the recent evolution of transcriptome profiling methods and the surprising insights that have emerged in recent years regarding distinct mRNA processing events - from the 5' end to the 3' end of the molecule.

A recap of RNA recapping

The N7-methylguanosine cap is a hallmark of the 5' end of eukaryotic mRNAs and is required for gene expression. Loss of the cap was believed to lead irreversibly to decay. However, nearly a decade ago, it was discovered that mammalian cells contain enzymes in the cytoplasm that are capable of restoring caps onto uncapped RNAs. In this review, we summarize recent advances in our understanding of cytoplasmic RNA recapping and discuss the biochemistry of this process and its impact on regulating and diversifying the transcriptome. Although most studies focus on mammalian RNA recapping, we also highlight new observations for recapping in disparate eukaryotic organisms, with the trypanosome recapping system appearing to be a fascinating example of convergent evolution. We conclude with emerging insights into the biological significance of RNA recapping and prospects for the future of this evolving area of study. This article is categorized under: RNA Processing > RNA Editing and Modification Translation > Translation Regulation RNA Processing > Capping and 5' End Modifications RNA Turnover and Surveillance > Regulation of RNA Stability.

RNA-binding proteins and heat-shock protein 90 are constituents of the cytoplasmic capping enzyme interactome

The N7-methylguanosine cap is added in the nucleus early in gene transcription and is a defining feature of eukaryotic mRNAs. Mammalian cells also possess cytoplasmic machinery for restoring the cap at uncapped or partially degraded RNA 5' ends. Central to both pathways is capping enzyme (CE) (RNA guanylyltransferase and 5'-phosphatase (RNGTT)), a bifunctional, nuclear and cytoplasmic enzyme. CE is recruited to the cytoplasmic capping complex by binding of a C-terminal proline-rich sequence to the third Src homology 3 (SH3) domain of NCK adapter protein 1 (NCK1). To gain broader insight into the cellular context of cytoplasmic recapping, here we identified the protein interactome of cytoplasmic CE in human U2OS cells through two complementary approaches: chemical cross-linking and recovery with cytoplasmic CE and protein screening with proximity-dependent biotin identification (BioID). This strategy unexpectedly identified 66 proteins, 52 of which are RNA-binding proteins. We found that CE interacts with several of these proteins independently of RNA, mediated by sequences within its N-terminal triphosphatase domain, and we present a model describing how CE-binding proteins may function in defining recapping targets. This analysis also revealed that CE is a client protein of heat shock protein 90 (HSP90). Nuclear and cytoplasmic CEs were exquisitely sensitive to inhibition of HSP90, with both forms declining significantly following treatment with each of several HSP90 inhibitors. Importantly, steady-state levels of capped mRNAs decreased in cells treated with the HSP90 inhibitor geldanamycin, raising the possibility that the cytotoxic effect of these drugs may partially be due to a general reduction in translatable mRNAs.

Nck1 Deficiency Impairs Adipogenesis by Activation of PDGFRα in Preadipocytes

Obesity results from an excessive expansion of white adipose tissue (WAT), which is still poorly understood from an etiologic-mechanistic perspective. Here, we report that Nck1, a Src homology domain-containing adaptor, is upregulated during WAT expansion and in vitro adipogenesis. In agreement, Nck1 mRNA correlates positively with peroxisome proliferator-activated receptor (PPAR) γ and adiponectin mRNAs in the WAT of obese humans, whereas Nck1-deficient mice display smaller WAT depots with reduced number of adipocyte precursors and accumulation of extracellular matrix. Furthermore, silencing Nck1 in 3T3-L1 preadipocytes increases the proliferation and expression of genes encoding collagen, whereas it decreases the expression of adipogenic markers and impairs adipogenesis. Silencing Nck1 in 3T3-L1 preadipocytes also promotes the expression of platelet-derived growth factor (PDGF)-A and platelet-derived growth factor receptor (PDGFR) α activation and signaling. Preventing PDGFRα activation using imatinib, or through PDGF-A or PDGFRα deficiency, inhibits collagen expression in Nck1-deficient preadipocytes. Finally, imatinib rescues differentiation of Nck1-deficient preadipocytes. Altogether, our findings reveal that Nck1 modulates WAT development through PDGFRα-dependent remodeling of preadipocytes.

Peptide-based sequestration of the adaptor protein Nck1 in pancreatic β cells enhances insulin biogenesis and protects against diabetogenic stresses

One feature of diabetes is the failure of pancreatic β cells to produce insulin, but the molecular mechanisms leading to this failure remain unclear. Increasing evidence supports a role for protein kinase R-like endoplasmic reticulum kinase (PERK) in the development and function of healthy pancreatic β cells. Previously, our group identified the adaptor protein Nck1 as a negative regulator of PERK. Indeed, we demonstrated that Nck1, by directly binding PERK autophosphorylated on Tyr⁵⁶¹, limits PERK activation and signaling. Accordingly, we found that stable depletion of Nck1 in β cells promotes PERK activation and signaling, increases insulin biosynthesis, and improves cell viability in response to diabetes-related stresses. Herein, we explored the therapeutic potential of abrogating the interaction between Nck and PERK to improve β-cell function and survival. To do so, we designed and used a peptide containing the minimal PERK sequence involved in binding Nck1 conjugated to the cell-permeable protein transduction domain from the HIV protein TAT. In the current study, we confirm that the synthetic TAT-Tyr(P)⁵⁶¹ phosphopeptide specifically binds the SH2 domain of Nck and prevents Nck interaction with PERK, thereby promoting basal PERK activation. Moreover, we report that treatment of β cells with TAT-Tyr(P)⁵⁶¹ inhibits glucolipotoxicity-induced apoptosis, whereas it enhances insulin production and secretion. Taken together, our results support the potential of sequestering Nck using a synthetic peptide to enhance basal PERK activation and create more robust β cells.

RNA guanine-7 methyltransferase catalyzes the methylation of cytoplasmically recapped RNAs

Cap homeostasis is a cyclical process of decapping and recapping that impacts a portion of the mRNA transcriptome. The metastable uncapped forms of recapping targets redistribute from polysomes to non-translating mRNPs, and recapping is all that is needed for their return to the translating pool. Previous work identified a cytoplasmic capping metabolon consisting of capping enzyme (CE) and a 5′-monophosphate kinase bound to adjacent domains of Nck1. The current study identifies the canonical cap methyltransferase (RNMT) as the enzyme responsible for guanine-N7 methylation of recapped mRNAs. RNMT binds directly to CE, and its presence in the cytoplasmic capping complex was demonstrated by pulldown assays, gel filtration and proximity-dependent biotinylation. The latter also identified the RNMT cofactor RAM, whose presence is required for cytoplasmic cap methyltransferase activity. These findings guided development of an inhibitor of cytoplasmic cap methylation whose action resulted in a selective decrease in levels of recapped mRNAs.

Regulation by 3'-Untranslated Regions

3'-untranslated regions (3'-UTRs) are the noncoding parts of mRNAs. Compared to yeast, in humans, median 3'-UTR length has expanded approximately tenfold alongside an increased generation of alternative 3'-UTR isoforms. In contrast, the number of coding genes, as well as coding region length, has remained similar. This suggests an important role for 3'-UTRs in the biology of higher organisms. 3'-UTRs are best known to regulate diverse fates of mRNAs, including degradation, translation, and localization, but they can also function like long noncoding or small RNAs, as has been shown for whole 3'-UTRs as well as for cleaved fragments. Furthermore, 3'-UTRs determine the fate of proteins through the regulation of protein-protein interactions. They facilitate cotranslational protein complex formation, which establishes a role for 3'-UTRs as evolved eukaryotic operons. Whereas bacterial operons promote the interaction of two subunits, 3'-UTRs enable the formation of protein complexes with diverse compositions. All of these 3'-UTR functions are accomplished by effector proteins that are recruited by RNA-binding proteins that bind to 3'-UTR cis-elements. In summary, 3'-UTRs seem to be major players in gene regulation that enable local functions, compartmentalization, and cooperativity, which makes them important tools for the regulation of phenotypic diversity of higher organisms.

African Swine Fever Virus NP868R Capping Enzyme Promotes Reovirus Rescue during Reverse Genetics by Promoting Reovirus Protein Expression, Virion Assembly, and RNA Incorporation into Infectious Virions

Reoviruses, like many eukaryotic viruses, contain an inverted 7-methylguanosine (m7G) cap linked to the 5' nucleotide of mRNA. The traditional functions of capping are to promote mRNA stability, protein translation, and concealment from cellular proteins that recognize foreign RNA. To address the role of mRNA capping during reovirus replication, we assessed the benefits of adding the African swine fever virus NP868R capping enzyme during reovirus rescue. C3P3, a fusion protein containing T7 RNA polymerase and NP868R, was found to increase protein expression 5- to 10-fold compared to T7 RNA polymerase alone while enhancing reovirus rescue from the current reverse genetics system by 100-fold. Surprisingly, RNA stability was not increased by C3P3, suggesting a direct effect on protein translation. A time course analysis revealed that C3P3 increased protein synthesis within the first 2 days of a reverse genetics transfection. This analysis also revealed that C3P3 enhanced processing of outer capsid μ1 protein to μ1C, a previously described hallmark of reovirus assembly. Finally, to determine the rate of infectious-RNA incorporation into new virions, we developed a new recombinant reovirus S1 gene that expressed the fluorescent protein UnaG. Following transfection of cells with UnaG and infection with wild-type virus, passage of UnaG through progeny was significantly enhanced by C3P3. These data suggest that capping provides nontraditional functions to reovirus, such as promoting assembly and infectious-RNA incorporation.IMPORTANCE Our findings expand our understanding of how viruses utilize capping, suggesting that capping provides nontraditional functions to reovirus, such as promoting assembly and infectious-RNA incorporation, in addition to enhancing protein translation. Beyond providing mechanistic insight into reovirus replication, our findings also show that reovirus reverse genetics rescue is enhanced 100-fold by the NP868R capping enzyme. Since reovirus shows promise as a cancer therapy, efficient reovirus reverse genetics rescue will accelerate production of recombinant reoviruses as candidates to enhance therapeutic potency. NP868R-assisted reovirus rescue will also expedite production of recombinant reovirus for mechanistic insights into reovirus protein function and structure.

Nck2 Deficiency in Mice Results in Increased Adiposity Associated With Adipocyte Hypertrophy and Enhanced Adipogenesis

Obesity results from an excessive expansion of white adipose tissue (WAT) from hypertrophy of preexisting adipocytes and enhancement of precursor differentiation into mature adipocytes. We report that Nck2-deficient mice display progressive increased adiposity associated with adipocyte hypertrophy. A negative relationship between the expression of Nck2 and WAT expansion was recapitulated in humans such that reduced Nck2 protein and mRNA levels in human visceral WAT significantly correlate with the degree of obesity. Accordingly, Nck2 deficiency promotes an adipogenic program that not only enhances adipocyte differentiation and lipid droplet formation but also results in dysfunctional elevated lipogenesis and lipolysis activities in mouse WAT as well as in stromal vascular fraction and 3T3-L1 preadipocytes. We provide strong evidence to support that through a mechanism involving primed PERK activation and signaling, Nck2 deficiency in adipocyte precursors is associated with enhanced adipogenesis in vitro and adiposity in vivo. Finally, in agreement with elevated circulating lipids, Nck2-deficient mice develop glucose intolerance, insulin resistance, and hepatic steatosis. Taken together, these findings reveal that Nck2 is a novel regulator of adiposity and suggest that Nck2 is important in limiting WAT expansion and dysfunction in mice and humans.

mRNA capping: biological functions and applications

The 5′ m7G cap is an evolutionarily conserved modification of eukaryotic mRNA. Decades of research have established that the m7G cap serves as a unique molecular module that recruits cellular proteins and mediates cap-related biological functions such as pre-mRNA processing, nuclear export and cap-dependent protein synthesis. Only recently has the role of the cap 2′O methylation as an identifier of self RNA in the innate immune system against foreign RNA has become clear. The discovery of the cytoplasmic capping machinery suggests a novel level of control network. These new findings underscore the importance of a proper cap structure in the synthesis of functional messenger RNA. In this review, we will summarize the current knowledge of the biological roles of mRNA caps in eukaryotic cells. We will also discuss different means that viruses and their host cells use to cap their RNA and the application of these capping machineries to synthesize functional mRNA. Novel applications of RNA capping enzymes in the discovery of new RNA species and sequencing the microbiome transcriptome will also be discussed. We will end with a summary of novel findings in RNA capping and the questions these findings pose.

The human PMR1 endonuclease stimulates cell motility by down regulating miR-200 family microRNAs

The motility of MCF-7 cells increases following expression of a human PMR1 transgene and the current study sought to identify the molecular basis for this phenotypic change. Ensemble and single cell analyses show increased motility is dependent on the endonuclease activity of hPMR1, and cells expressing active but not inactive hPMR1 invade extracellular matrix. Nanostring profiling identified 14 microRNAs that are downregulated by hPMR1, including all five members of the miR-200 family and others that also regulate invasive growth. miR-200 levels increase following hPMR1 knockdown, and changes in miR-200 family microRNAs were matched by corresponding changes in miR-200 targets and reporter expression. PMR1 preferentially cleaves between UG dinucleotides within a consensus YUGR element when present in the unpaired loop of a stem–loop structure. This motif is present in the apical loop of precursors to most of the downregulated microRNAs, and hPMR1 targeting of pre-miRs was confirmed by their loss following induced expression and increase following hPMR1 knockdown. Introduction of miR-200c into hPMR1-expressing cells reduced motility and miR-200 target gene expression, confirming hPMR1 acts upstream of Dicer processing. These findings identify a new role for hPMR1 in the post-transcriptional regulation of microRNAs in breast cancer cells.

Cap homeostasis is independent of poly(A) tail length

Cap homeostasis is a cyclical process of decapping and recapping that maintains the cap on a subset of the cytoplasmic transcriptome. Interfering with cytoplasmic capping results in the redistribution of target transcripts from polysomes to non-translating mRNPs, where they accumulate in an uncapped but nonetheless stable form. It is generally thought that decapping is preceded by shortening of the poly(A) tail to a length that can no longer support translation. Therefore recapped target transcripts would either have to undergo cytoplasmic polyadenylation or retain a reasonably long poly(A) tail if they are to return to the translating pool. In cells that are inhibited for cytoplasmic capping there is no change in the overall distribution of poly(A) lengths or in the elution profile of oligo(dT)-bound targets. Poly(A) tail lengths were similar for target mRNAs on polysomes or in non-translating mRNPs, and the presence of polyadenylated uncapped mRNA in mRNPs was confirmed by separation into capped and uncapped pools prior to assay. Finally, in silico analysis of cytoplasmic capping targets revealed significant correlations with genes encoding transcripts with uridylated or multiply modified 3′ ends, and genes possessing multiple 3′-untranslated regions (UTRs) generated by alternative cleavage and polyadenylation.

Centrin 3 is an inhibitor of centrosomal Mps1 and antagonizes centrin 2 function

Centrins are a family of small, calcium-binding proteins with diverse cellular functions that play an important role in centrosome biology. We previously identified centrin 2 and centrin 3 (Cetn2 and Cetn3) as substrates of the protein kinase Mps1. However, although Mps1 phosphorylation sites control the function of Cetn2 in centriole assembly and promote centriole overproduction, Cetn2 and Cetn3 are not functionally interchangeable, and we show here that Cetn3 is both a biochemical inhibitor of Mps1 catalytic activity and a biological inhibitor of centrosome duplication. In vitro, Cetn3 inhibits Mps1 autophosphorylation at Thr-676, a known site of T-loop autoactivation, and interferes with Mps1-dependent phosphorylation of Cetn2. The cellular overexpression of Cetn3 attenuates the incorporation of Cetn2 into centrioles and centrosome reduplication, whereas depletion of Cetn3 generates extra centrioles. Finally, overexpression of Cetn3 reduces Mps1 Thr-676 phosphorylation at centrosomes, and mimicking Mps1-dependent phosphorylation of Cetn2 bypasses the inhibitory effect of Cetn3, suggesting that the biological effects of Cetn3 are due to the inhibition of Mps1 function at centrosomes.

Uncapped 5' ends of mRNAs targeted by cytoplasmic capping map to the vicinity of downstream CAGE tags

In mammalian transcriptomes approximately 25% of 5' ends determined by Capped Analysis of Gene Expression (CAGE) map to locations within spliced exons. The current study sought to determine if the cytoplasmic capping complex participates in generating these downstream CAGE tags. 5'-RACE was used to amplify the uncapped ends of target transcripts that accumulate when cytoplasmic capping is blocked. Sequencing of these RACE products mapped the positions of uncapped ends either exactly to or just downstream of archived CAGE tags. These findings support a role for cytoplasmic capping in generating the downstream capped ends identified by CAGE.

Nck1 depletion induces activation of the PI3K/Akt pathway by attenuating PTP1B protein expression

Background

Activation of the PI3K/Akt pathway mediates crucial cellular functions regulated by receptor tyrosine kinases, such as cell growth, proliferation, survival and metabolism. Previously, we reported that the whole-body knockout of the Src homology domain-containing adaptor protein Nck1 improves overall glucose homeostasis and insulin-induced activation of the PI3K/Akt pathway in liver of obese mice. The aim of the current study is to elucidate the mechanism by which Nck1 depletion regulates hepatic insulin signaling.

Results

Here, we demonstrate that Nck1 regulates the activation of the PI3K/Akt pathway in a protein tyrosine phosphatase 1B (PTP1B)-dependent mechanism. Indeed, depletion of Nck1 by siRNA in HepG2 cells enhances PI3K-dependent basal and growth factor-induced Akt activation. In accordance, primary hepatocytes isolated from Nck1^−/− mice also display enhanced Akt activation in response to insulin. Activation of the PI3K/Akt pathway in Nck1-depleted HepG2 cells relies on higher levels of tyrosine-phosphorylated proteins and correlates with decreased PTP1B levels. Interestingly, Nck1 and PTP1B in cells are found in a common molecular complex and their interaction is dependent on the SH3 domains of Nck1. Finally, Nck1 depletion in HepG2 cells neither affects PTP1B gene transcription nor PTP1B protein stability, suggesting that Nck1 modulates PTP1B expression at the translational level.

Conclusion

Our study provides strong evidence supporting that the adaptor protein Nck1 interacts with PTP1B and also regulates PTP1B expression. In this manner, Nck1 plays a role in regulating the PI3K/Akt pathway.