Stem-loop Binding Protein Research Articles

Uridylation of the histone mRNA stem-loop weakens binding interactions with SLBP while maintaining interactions with 3’hExo

ABSTRACT Histone mRNA degradation is controlled by the unique 3’ stem-loop of histone mRNA and the stem-loop binding protein (SLBP). As part of this process, the 3’ stem-loop is trimmed by the histone-specific 3’ exonuclease (3’hExo) and uridylated by the terminal uridylyl transferase 7 (TUT7), creating partially degraded intermediates with short uridylations. The role of these uridylations in degradation is not fully understood. Our work examines changes in the stability of the ternary complex created by trimming and uridylation of the stem-loop to better understand the role of this process in the histone mRNA life cycle. In this study, we used fluorescence polarization and electrophoretic mobility shift assays to demonstrate that both SLBP and 3’hExo can bind to uridylated and partially degraded stem-loop intermediates, although with lower affinity. We further characterized this complex by performing 1-µs molecular dynamics simulations using the AMBER force field and Nanoscale Molecular Dynamics (NAMD). These simulations show that while uridylation helps maintain the overall shape of the stem-loop, the combination of uridylation and dephosphorylation of the TPNK motif in SLBP disrupts key RNA–protein interactions. They also demonstrate that uridylation allows 3’hExo to maintain contact with the stem-loop after partial degradation and plays a role in disrupting key base pairs in partially degraded histone mRNA intermediates. Together, these experiments and simulations suggest that trimming by 3’hExo, uridylation, and SLBP dephosphorylation weakens both RNA–protein interactions and the stem-loop itself. Our results further elucidate the role of uridylation and SLBP dephosphorylation in the early stages of histone mRNA degradation.

Nsp14 of SARS-CoV-2 inhibits mRNA processing and nuclear export by targeting the nuclear cap-binding complex.

To facilitate selfish replication, viruses halt host gene expression in various ways. The nuclear export of mRNA is one such process targeted by many viruses. SARS-CoV-2, the etiological agent of severe acute respiratory syndrome, also prevents mRNA nuclear export. In this study, Nsp14, a bifunctional viral replicase subunit, was identified as a novel inhibitor of mRNA nuclear export. Nsp14 induces poly(A)+ RNA nuclear accumulation and the dissolution/coalescence of nuclear speckles. Genome-wide gene expression analysis revealed the global dysregulation of splicing and 3'-end processing defects of replication-dependent histone mRNAs by Nsp14. These abnormalities were also observed in SARS-CoV-2-infected cells. A mutation introduced at the guanine-N7-methyltransferase active site of Nsp14 diminished these inhibitory activities. Targeted capillary electrophoresis-mass spectrometry analysis (CE-MS) unveiled the production of N7-methyl-GTP in Nsp14-expressing cells. Association of the nuclear cap-binding complex (NCBC) with the mRNA cap and subsequent recruitment of U1 snRNP and the stem-loop binding protein (SLBP) were impaired by Nsp14. These data suggest that the defects in mRNA processing and export arise from the compromise of NCBC function by N7-methyl-GTP, thus exemplifying a novel viral strategy to block host gene expression.

Electronegative clusters modulate folding status and RNA binding of unstructured RNA-binding proteins.

Electronegative clusters (ENCs) made up of acidic residues and/or phosphorylation sites are the most abundant repetitive sequences in RNA-binding proteins. Previous studies have indicated that ENCs inhibit RNA binding for structured RNA-binding domains (RBDs). However, this is not the case for the unstructured RBD in histone pre-mRNA stem-loop binding protein (SLBP). The SLBP RBD contains 70 amino acids and is followed by a phosphorylatable ENC. ENC phosphorylation increases RNA-binding affinity of SLBP to the sub-picomolar range. In this study, we use NMR and molecular dynamics simulations to elucidate the mechanism for this tight binding. Our NMR data demonstrate that the ENC transiently folds apo SLBP into an RNA-bound resembling state. We find that in the RNA-bound state, the phosphorylated ENC interacts with the loop region opposite to the RNA-binding site. This allosteric interaction stabilizes the complex and therefore enhances RNA binding. To evaluate the generality of our findings, we graft an ENC onto endoribonuclease homolog 1's first double-stranded RNA-binding motif (DRBM1), an unstructured RBD that shares no homology with SLBP. We find that the engineered ENC increases the folded species of DRBM1 and inhibits RNA binding. On the contrary, introducing basic residues to DRBM1 makes the domain more unfolded, enhances RNA binding, and mitigates the inhibitory effect of the engineered ENC. In summary, our study suggests that ENCs promote folding of unstructured RNA-binding domains, and their effects on RNA binding depend on the electropositive charges on the RBD surface.

An increase in polyadenylation of histone isoforms, Hist1h2ah and Hist2h3c2, is governed by 3'-UTR in de-differentiated and undifferentiated hepatocyte.

Replication-dependent histones have a stem-loop structure at the 3' end of messenger RNA (mRNA) and are stabilized by stem-loop binding protein (SLBP). Moreover, loss of SLBP and imbalance in the level of ARE (adenylate-uridylate-rich elements)-binding proteins, HuR, and BRF1 are associated with the polyadenylation of canonical histone mRNAs under different physiological conditions. Previous studies from the lab have shown increased protein levels of H2A1H and H3.2 in N-nitrosodiethylamine (NDEA)-induced hepatocellular carcinoma (HCC). In this study, we report that increase in the polyadenylation of histone mRNA contributes to increased levels of H2A1H and H3.2 in NDEA-induced HCC. The persistent exposure to carcinogen with polyadenylation of histone mRNA increases the total histone pool resulting in aneuploidy. The embryonic liver has also shown increased polyadenylated histone isoforms, Hist1h2ah and Hist2h3c2, primarily contributing to their increased protein levels. The increase in polyadenylation of histone mRNA in HCC and e15 are in coherence with the decrease in SLBP and BRF1 with an increase in HuR. Our studies in neoplastic CL38 cell line showed that direct stress on the cells induces downregulation of SLBP with enhanced histone isoform polyadenylation. Moreover, the polyadenylation is related to increase in activated MAP kinases, p38, ERK, and JNK in HCC liver tumor tissues and CL38 cells treated with arsenic. Our data suggest that SLBP degrades under stress, destabilizing the stem-loop, elongating histone isoforms mRNA with 3' polyadenylated tail with increase of HuR and decrease of BRF1. Overall, our results indicate that SLBP may play an essential part in cell proliferation, at least in persistent exposure to stress, by mediating the stabilization of histone isoforms throughout the cell cycle.

Polyadenylation of canonical histone H3.1 in carcinogenesis.

Canonical histone messenger RNAs (mRNAs) are transcribed during S phase and do not terminate with a poly(A) tail at the 3' end. Instead, the histone mRNAs display a stem-loop structure at their 3-end. Stem-loop-binding protein (SLBP) binds the stem-loop and regulates canonical histone mRNA metabolism. We previously demonstrated that exposure to arsenic, an environmental carcinogen, induces polyadenylation of canonical histone H3.1 mRNA, causing transformation of human cells in vitro. Arsenic decreased cellular levels of SLBP by inducing its proteasomal degradation and inhibiting SLBP transcription via epigenetic mechanisms. Similarly, we also reported that nickel and arsenic have similar effects on canonical histone mRNA transcription and translation. Most recently, we further demonstrated that bisphenols' exposure increased polyadenylation of canonical histone H3.1 mRNA possibly through down-regulation of SLBP expression. This facilitates the abnormal stability of at least one canonical histone isoform (H3.1), and also increases H3 protein levels. Excess expression of canonical histones have been shown to increase sensitivity to DNA damage as well as increase the frequency of missing chromosomes and induce genomic instability. Thus, polyadenylation of canonical histone mRNA following arsenic, nickel and bisphenols exposure may contribute to metal and bisphenol-induced carcinogenesis.

Integrated proteomic and phosphoproteomic analysis for characterization of colorectal cancer

Proteins and translationally modified proteins like phosphoproteins have essential regulatory roles in tumorigenesis. This study attempts to elucidate the dysregulated proteins driving colorectal cancer (CRC). To explore the differential proteins, we performed iTRAQ labeling proteomics and TMT labeling phosphoproteomics analysis of CRC tissues and adjacent non-cancerous tissues. The functions of quantified proteins were analyzed using Gene Ontology (GO), the Kyoto Encyclopedia of Genes and Genomes (KEGG), and Subcellular localization analysis. Depending on the results, we identified 330 differential proteins and 82 phosphoproteins in CRC. GO and KEGG analyses demonstrated that protein changes were primarily associated with regulating biological and metabolic processes through binding to other molecules. Co-expression relationships between proteomic and phosphoproteomic analysis revealed that TMC5, SMC4, SLBP, VSIG2, and NDRG2 were significantly dysregulated differential proteins. Additionally, based on the predicted co-expression proteins, we identified that the stem-loop binding protein (SLBP) was up-regulated in CRC cells and promoted the proliferation and migration of CRC. This study reports an integrated proteomic and phosphoproteomic analysis of CRC to discern the functional impact of protein alterations and provides a candidate diagnostic biomarker or therapeutic target for CRC. SignificanceCombining one or more high-throughput omics technologies with bioinformatics to analyze biological samples and explore the links between biomolecules and their functions can provide more comprehensive and multi-level insights for disease mechanism research. Proteomics, phosphoproteomics, metabolomics and their combined analysis play an important role in the auxiliary diagnosis, the discovery of biomarkers and novel therapeutic targets for colorectal cancer. In this integrated proteomic and phosphoproteomic analysis, we identified proteins and phosphoproteins in colorectal cancer tissue and analyzed potential mechanisms contributing to progression in colorectal cancer. The results of this study provide a foundation to focus future experiments on the contribution of altered protein and phosphorylation patterns to prevention and treatment of colorectal cancer.

Downregulation of Stem-Loop Binding Protein by Nicotine via α7-Nicotinic Acetylcholine Receptor and Its Role in Nicotine-Induced Cell Transformation.

The use of electronic-cigarettes (e-cigs) has increased substantially in recent years, particularly among the younger generations. Liquid nicotine is the main component of e-cigs. Previous studies have shown that mice exposed to e-cig aerosols developed lung adenocarcinoma and bladder hyperplasia. These findings implicated a potential role for e-cig aerosols and nicotine in cancer development, although the underlying mechanisms are not fully understood. Here we report that exposure to liquid nicotine or nicotine aerosol generated from e-cig induces downregulation of Stem-loop binding protein (SLBP) and polyadenylation of canonical histone mRNAs in human bronchial epithelial cells and in mice lungs. Canonical histone mRNAs typically do not end in a poly(A) tail and the acquisition of such a tail via depletion of SLBP has been shown to causes chromosome instability. We show that nicotine-induced SLBP depletion is reversed by an inhibitor of α7-nicotinic acetylcholine receptors (α7-nAChR) or siRNA specific for α7-nAChR, indicating a nAChR-dependent reduction of SLBP by nicotine. Moreover, PI3K/AKT pathway is activated by nicotine exposure and CK2 and probably CDK1, 2 kinases well known for their function for SLBP phosphorylation and degradation, are shown to be involved, α7-nAChR-dependently, in nicotine-induced SLBP depletion. Importantly, nicotine-induced anchorage-independent cell growth is attenuated by inhibition of α7-nAChR and is rescued by overexpression of SLBP. We propose that the SLBP depletion and polyadenylation of canonical histone mRNAs via activation of α7-nAChR and a series of downstream signal transduction pathways are critical for nicotine-induced cell transformation and potential carcinogenesis.

Abstract 228: Depletion of stem-loop binding protein and elevation of polyadenylation of histone 3.1 contribute to malignant cell transformation induced by bisphenol A

Abstract Introduction: Bisphenol A (BPA) is a pervasive chemical exposure in the US and has been linked to adverse health effects in humans and mainly identified as an endocrine disruptor. However, recent studies suggests that all plastics including our daily usages for food/drink containers may leach chemicals if they are scratched or heated or stored long term and strongly suggests that at certain exposure levels of BPA in these products, may cause cancers including breast, lung, and ovarian cancers in humans. Although there are few studies about breast, lung and ovarian cancers, with BPA exposure there is an urgent need to focus on the BPA’s carcinogenic activity in the lung. Because lung cancers can progress to life threatening events and short life span once it is diagnosed, identifying the mechanism that specifically inhibits/prevents lung cancers invasion and related morbidity is of tremendous importance for reducing mortality. Therefore, in this study, we investigated the carcinogenicity of BPA using human bronchial epithelial cells. Experimental methods: To determine whether BPA alters canonical histone mRNA processing in vitro, we measured levels of polyadenylation (polyA) of canonical histone (H)3.1 mRNA as well as protein expression of Stem-loop binding protein (SLBP), and H3 protein in human bronchial epithelial BEAS-2B cells exposed to various concentrations of BPA for 96 h. Furthermore, BEAS-2B cells were chronically exposed to low doses of BPA, the cell transformation assay was performed in an anchorage-independent growth and colony formation was assessed. Arsenic was used as a positive control because our laboratory previously demonstrated that arsenic-induced polyA of H3.1 mRNA, by depletion of SLBP in vitro and that polyA of H3.1 mRNA enhanced tumor formation in nude mice. Results: Our results showed that exposure of BEAS-2B cells to BPA depleted SLBP, induced polyA of canonical histone mRNAs, and upregulated H3 protein. The results from real-time quantitative PCR showed that BPA exposure increased level of polyA H3.1 mRNA. Furthermore, our findings from an anchorage-independent growth assay also indicate that exposure of BEAS-2B cells to low doses of BPA induced malignant cell transformation. Conclusions: The present study demonstrated that chronic exposure of the cells to BPA causes malignant cell transformation, indicating that BPA is a potent human carcinogen to induce lung cancer. Our study also demonstrated that the loss of SLBP and gain of polyA H3.1 mRNA play an important role in BPA-induced carcinogenesis. Citation Format: Arul Veerappan, Zhou Zhang, Max Costa. Depletion of stem-loop binding protein and elevation of polyadenylation of histone 3.1 contribute to malignant cell transformation induced by bisphenol A [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 228.

Computational and Biophysical Analysis of RNA/Protein Complexes in Histone mRNA Degradation

Histone mRNA is the only known metazoan mRNA that is not polyadenylated at its 3’ end; instead, it ends in a highly conserved stem‐loop. This stem‐loop and its protein binding partner, the stem‐loop binding protein (SLBP), control every aspect of histone mRNA metabolism. As the cell prepares to degrade histone mRNA in the cytoplasm towards the end of S‐phase, both of these elements are modified; the mRNA is trimmed by 3’ human exonuclease (3’hExo) and uridylated by a terminal uridylyl transferase, while SLBP is dephosphorylated. The trimming and uridylation process creates uridylated intermediates that differ in length as the histone mRNA begins to undergo degradation, while the dephosphorylation of SLBP destabilizes the protein. Both of these events are associated with histone mRNA degradation, their exact roles in histone mRNA metabolism are not fully understood. In this study, we seek to characterize how these changes to SLBP and histone mRNA affect the stability of the ternary SLBP/3’hEXO/mRNA complex using both computational and experimental methods. To computationally characterize the effects of uridylation on stability of the ternary complex, we performed molecular dynamics simulations (1 µs) using AMBER force fields and Nanoscale Molecular Dynamics (NAMD). To experimentally characterize these observations, we used a fluorescence‐based EMSA competition assay to measure the proteins’ difference in affinity for the wild‐type stem‐loop and uridylated intermediates. Through these experiments, we have gained valuable insight into the role of both the uridylation of histone mRNA and the dephosphorylation of SLBP at the beginning of histone mRNA degradation.

Human cytomegalovirus lytic infection inhibits replication-dependent histone synthesis and requires stem loop binding protein function

Replication-dependent (RD) histones are deposited onto human cytomegalovirus (HCMV) genomes at the start of infection. We examined how HCMV affects the de novo production of RD histones and found that viral infection blocked the accumulation of RD histone mRNAs that normally occurs during the S phase. Furthermore, RD histone mRNAs present in HCMV-infected cells did not undergo the unique 3′ processing required for their normal nuclear export and translation. The protein that orchestrates processing in the nucleus, stem loop–binding protein (SLBP), was found predominantly in the cytoplasm, and RD histone proteins were not de novo synthesized in HCMV-infected cells. Intriguingly, however, we found that SLBP was required for the efficient synthesis and assembly of infectious progeny virions. We conclude that HCMV infection attenuates RD histone mRNA accumulation and processing and the de novo protein synthesis of the RD histones, while utilizing SLBP for an alternative purpose to support infectious virion production.

53BP1-ACLY-SLBP-coordinated activation of replication-dependent histone biogenesis maintains genomic integrity.

p53-binding protein 1 (53BP1) regulates the DNA double-strand break (DSB) repair pathway and maintains genomic integrity. Here we found that 53BP1 functions as a molecular scaffold for the nucleoside diphosphate kinase-mediated phosphorylation of ATP-citrate lyase (ACLY) which enhances the ACLY activity. This functional association is critical for promoting global histone acetylation and subsequent transcriptome-wide alterations in gene expression. Specifically, expression of a replication-dependent histone biogenesis factor, stem-loop binding protein (SLBP), is dependent upon 53BP1-ACLY-controlled acetylation at the SLBP promoter. This chain of regulation events carried out by 53BP1, ACLY, and SLBP is crucial for both quantitative and qualitative histone biogenesis as well as for the preservation of genomic integrity. Collectively, our findings reveal a previously unknown role for 53BP1 in coordinating replication-dependent histone biogenesis and highlight a DNA repair-independent function in the maintenance of genomic stability through a regulatory network that includes ACLY and SLBP.

Stem-loop binding protein and metal carcinogenesis.

Pre-mRNA processing of the replication-dependent canonical histone mRNAs requires an endonucleolytic cleavage immediately after a conserved stem loop structure which occurs before RNA Pol II encounters any poly(A) signal. Thus, in contrast to all other eukaryotic mRNAs, the canonical histone mRNAs are not polyadenylated in their 3' ends. The binding of stem-loop binding protein (SLBP) to the stem loop structure of the histone mRNAs is required for this process. SLBP is also involved in regulation of histone mRNA nuclear export, degradation, and translation. Depletion of SLBP has been shown to induce polyadenylation of histone mRNAs and alteration of histone protein levels, which are considered to contribute to the observed aberrant cell cycle progress and genomic instability resulting from the loss of SLBP function. Recent studies have demonstrated that some heavy metal carcinogens, including arsenic and nickel, can induce the loss of SLBP and the gain of polyadenylation of canonical histone mRNAs. Polyadenylated canonical histone H3 can result in abnormal transcription, cell cycle arrest, genomic instability, and cell transformation, which links SLBP depletion and subsequent histone mRNA misprocessing to cancer. This review seeks to briefly summarize what is known about regulation of SLBP expression, consequences of SLBP depletion, its roles in cancer-related end points, with particular focus on metal-induced SLBP depletion and the potential of SLBP depletion as a new mechanism for metal-induced carcinogenesis.

DRAIC promotes growth of breast cancer by sponging miR-432-5p to upregulate SLBP.

Mounting evidence suggests that lncRNAs can exert functions in cancer progression in multiple manners. In recent years, competing endogenous RNA (ceRNA) has been widely reported in human cancers as a lncRNA-dominant molecular pathway. The current study aimed at proving the role of lncRNA downregulated RNA in cancer (DRAIC) in breast cancer (BRCA) progression. To be specific, qRT-PCR assay was conducted to measure the expression of DRAIC and other downstream target genes. It was uncovered that DRAIC was expressed at a high level in BRCA cells. Functional analyses, including CCK-8, colony formation, and EdU assays demonstrated that DRAIC depletion suppressed BRCA cell proliferation. In addition, cell apoptosis was promoted due to DRAIC knockdown. The inhibitory effect of DRAIC reduction on BRCA cell migration and invasion was proven by transwell assays. Mechanistically, DRAIC was confirmed to predominantly distribute in the cytoplasm and could interact with miR-432-5p. In addition, stem-loop binding protein (SLBP) was verified to be a downstream target of miR-432-5p and was positively regulated by DRAIC. Taken together, DRAIC sponged miR-432-5p to enhance SLBP expression, by which malignant behaviors of BRCA cells were promoted. Our findings may help to provide a promising therapeutic target for BRCA patients.

Abstract LB191: Downregulation of Stem-loop binding protein by nicotine and its role in nicotine-induced cell transformation in human lung epithelial cells

Abstract The use of electronic-cigarettes has increased substantially in recent years among the younger population in particular. The liquid nicotine is the main component of the e-cigarette. Mice exposed to e-cigarette smoke (ECS) generated by heating the liquid nicotine developed lung adenocarcinoma and bladder hyperplasia. This implicated a potential role for ECS/nicotine in cancer development, while the underlying mechanisms remain unclear. Here we report that nicotine reduces the protein level of Stem-loop binding protein (SLBP), a critical factor for the stability, processing and translation of canonical histone pre-mRNAs, in human lung epithelial BEAS-2B cells. It has been known that canonical histone mRNAs typically do not end in a poly(A) tail and that depletion of SLBP results in polyadenylation of canonical histone mRNAs that appeared to be carcinogenic. Indeed, nicotine exposure induced polyadenylation of mRNAs for histones H2A, H2B, H3.1, H3.2, and H4. Interestingly, the inhibitor for nAChRs (nicotinic acetylcholine receptors) reversed nicotine-induced SLBP depletion. Moreover, the depletion of SLBP was attenuated not only by both the inhibitors and siRNAs specific for CDK1/2 and CK2, but also by inhibition of AKT signal pathway. Importantly, nicotine-induced anchorage-independent cell growth was rescued by overexpression of SLBP. These results suggest that activation of nAChR and a series of signal transduction pathways are involved in nicotine-mediated SLBP reduction and they may play an important role in nicotine-induced cell transformation and carcinogenesis.Keywords: Nicotine, Stem-loop binding protein, Cell transformation, nAChR Citation Format: Qi Sun, Danqi Chen, Chunyuan Jin. Downregulation of Stem-loop binding protein by nicotine and its role in nicotine-induced cell transformation in human lung epithelial cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB191.

Quantitative proteomics reveals arsenic attenuates stem‐loop binding protein stability via a chaperone complex containing heat shock proteins and ERp44

Arsenic pollution impacts health of millions of people in the world. Inorganic arsenic is a carcinogenic agent in skin and lung cancers. The stem-loop binding protein (SLBP) binds to the stem-loop of the canonical histone mRNA and regulates its metabolism during cell cycle. Our previous work has shown arsenic induces ubiquitin-proteasome dependent degradation of SLBP and contributes to lung cancer. In this study, we established the first comprehensive SLBP interaction network by affinity purification-mass spectrometry (AP-MS) analysis, and further demonstrated arsenic enhanced the association between SLBP and a crucial chaperone complex containing heat shock proteins (HSPs) and ERp44. Strikingly, knockdown of these proteins markedly rescued the protein level of SLBP under arsenic exposure conditions, and abolished the increasing migration capacity of BEAS-2B cells induced by arsenic. Taken together, our study provides a potential new mechanism that a chaperone complex containing HSPs and ERp44 attenuates the stability of SLBP under both normal and arsenic exposure conditions, which could be essential for arsenic-induced high cell migration.

StoatyDive: Evaluation and classification of peak profiles for sequencing data.

BackgroundThe prediction of binding sites (peak-calling) is a common task in the data analysis of methods such as cross-linking immunoprecipitation in combination with high-throughput sequencing (CLIP-Seq). The predicted binding sites are often further analyzed to predict sequence motifs or structure patterns. When looking at a typical result of such high-throughput experiments, the obtained peak profiles differ largely on a genomic level. Thus, a tool is missing that evaluates and classifies the predicted peaks on the basis of their shapes. We hereby present StoatyDive, a tool that can be used to filter for specific peak profile shapes of sequencing data such as CLIP.FindingsWith StoatyDive we are able to classify peak profile shapes from CLIP-seq data of the histone stem-loop-binding protein (SLBP). We compare the results to existing tools and show that StoatyDive finds more distinct peak shape clusters for CLIP data. Furthermore, we present StoatyDive’s capabilities as a quality control tool and as a filter to pick different shapes based on biological or technical questions for other CLIP data from different RNA binding proteins with different biological functions and numbers of RNA recognition motifs. We finally show that proteins involved in splicing, such as RBM22 and U2AF1, have potentially sharper-shaped peaks than other RNA binding proteins.ConclusionStoatyDive finally fills the demand for a peak shape clustering tool for CLIP-Seq data that fine-tunes downstream analysis steps such as structure or sequence motif predictions and that acts as a quality control.

Depletion of Stem‐loop binding protein and polyadenylation of canonical histone mRNAs by nicotine

The use of electronic-cigarettes has increased substantially in recent years among the younger population in particular. The liquid nicotine is the main component of the e-cigarette. Mice exposed to e-cigarette smoke (ECS) generated by heating the liquid nicotine developed lung adenocarcinoma and bladder hyperplasia. This implicated a potential role for ECS/nicotine in cancer development, while the underlying mechanisms remain unclear. Here we report that nicotine reduces the protein level of Stem-loop binding protein (SLBP), a critical factor for the stability, processing and translation of canonical histone pre-mRNAs, in human lung epithelial BEAS-2B cells. It has been known that canonical histone mRNAs typically do not end in a poly(A) tail and that depletion of SLBP results in polyadenylation of canonical histone mRNAs that appeared to be carcinogenic. Indeed, nicotine exposure induced polyadenylation of mRNAs for histones H2A, H2B, H3.1, H3.2, and H4. Interestingly, the inhibitor for nAChRs (nicotinic acetylcholine receptors) reversed nicotine-induced SLBP depletion. Moreover, the depletion of SLBP was attenuated not only by both the inhibitors and siRNAs specific for CDK1/2 and CK2, but also by inhibition of AKT signal pathway. Importantly, nicotine-induced anchorage-independent cell growth was rescued by overexpression of SLBP. These results suggest that activation of nAChR and a series of signal transduction pathways are involved in nicotine-mediated SLBP reduction and they may play an important role in nicotine-induced cell transformation and carcinogenesis.

Biophysical Characterization and Computational Modeling of Histone mRNA Degradation Intermediates

Histone mRNA is the only known metazoan mRNA that is not polyadenylated at its 3′ end; instead, it ends in a highly conserved stem-loop formed by Watson-Crick base pairing. In histone mRNA degradation, the stem-loop is trimmed by 3′ human exonuclease (3′hExo) and uridylated by terminal uridylyl transferase 7. This process creates uridylated intermediates—some of which are more common in the cell than expected. It is unknown how this trimming and uridylation affects the binding interactions between the stem-loop and its protein binding partners, the stem-loop binding protein (SLBP) and 3′hExo. In this work, we characterized the binding interactions of a wild-type stem-loop, a uridylated intermediate, and a truncated stem-loop using a combination of experimental and computational biophysical techniques. The uridylated intermediate represents one of the most prominent degradation intermediates identified in the cell, and the truncated stem-loop represents the stem-loop post-trimming and pre-uridylation. Through native electrophoretic mobility shift assays, we show that SLBP and 3′hExo are able to bind both the uridylated intermediate and the truncated stem-loop despite 3′ end modification. We also performed molecular dynamics simulations (1 µs) to evaluate structural changes induced by stem-loop modification and to understand how these changes impact SLBP and 3′hExo binding. RNA-protein systems were generated by applying AMBER force fields to modified versions of the histone mRNA ternary complex crystal structures (PDB 4QOZ and 4L8R). This work shows that modification of the stem-loop through trimming and uridylation does not interfere with the tertiary structure of the ternary complex.

A region of SLBP outside the mRNA-processing domain is essential for deposition of histone mRNA into the Drosophila egg.

Replication-dependent histone mRNAs are the only cellular mRNAs that are not polyadenylated, ending in a stemloop instead of a polyA tail, and are normally regulated coordinately with DNA replication. Stemloop-binding protein (SLBP) binds the 3' end of histone mRNA, and is required for processing and translation. During Drosophila oogenesis, large amounts of histone mRNAs and proteins are deposited in the developing oocyte. The maternally deposited histone mRNA is synthesized in stage 10B oocytes after the nurse cells complete endoreduplication. We report that in wild-type stage 10B oocytes, the histone locus bodies (HLBs), formed on the histone genes, produce histone mRNAs in the absence of phosphorylation of Mxc, which is normally required for histone gene expression in S-phase cells. Two mutants of SLBP, one with reduced expression and another with a 10-amino-acid deletion, fail to deposit sufficient histone mRNA in the oocyte, and do not transcribe the histone genes in stage 10B. Mutations in a putative SLBP nuclear localization sequence overlapping the deletion phenocopy the deletion. We conclude that a high concentration of SLBP in the nucleus of stage 10B oocytes is essential for histone gene transcription.This article has an associated First Person interview with the first author of the paper.

Molecular genetics of maternally-controlled cell divisions.

Forward genetic screens remain at the forefront of biology as an unbiased approach for discovering and elucidating gene function at the organismal and molecular level. Past mutagenesis screens targeting maternal-effect genes identified a broad spectrum of phenotypes ranging from defects in oocyte development to embryonic patterning. However, earlier vertebrate screens did not reach saturation, anticipated classes of phenotypes were not uncovered, and technological limitations made it difficult to pinpoint the causal gene. In this study, we performed a chemically-induced maternal-effect mutagenesis screen in zebrafish and identified eight distinct mutants specifically affecting the cleavage stage of development and one cleavage stage mutant that is also male sterile. The cleavage-stage phenotypes fell into three separate classes: developmental arrest proximal to the mid blastula transition (MBT), irregular cleavage, and cytokinesis mutants. We mapped each mutation to narrow genetic intervals and determined the molecular basis for two of the developmental arrest mutants, and a mutation causing male sterility and a maternal-effect mutant phenotype. One developmental arrest mutant gene encodes a maternal specific Stem Loop Binding Protein, which is required to maintain maternal histone levels. The other developmental arrest mutant encodes a maternal-specific subunit of the Minichromosome Maintenance Protein Complex, which is essential for maintaining normal chromosome integrity in the early blastomeres. Finally, we identify a hypomorphic allele of Polo-like kinase-1 (Plk-1), which results in a male sterile and maternal-effect phenotype. Collectively, these mutants expand our molecular-genetic understanding of the maternal regulation of early embryonic development in vertebrates.