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Arginine methylated peptides identified by SILAC-proteomics

This file contains additional data supporting the role of Prmt1 in human OS maintenance and murine OS tumor initiation, as well as proteomics and RNA-seq data supporting the role of Prmt1 in translation regulation.

GSEA analysis of differentially expressed genes in the control versus Prmt1 KO mOS at the transcript level using hallmark gene sets.

Function enrichment of down regulated genes upon Prmt1 depletion at the translation level using DAVID functional annotation tool (GO BP_direct).

Through an shRNA screen, we identified the protein arginine methyltransferase Prmt1 as a vulnerable intervention point in murine p53/Rb-null osteosarcomas, the human counterpart of which lacks effective therapeutic options. Depletion of Prmt1 in p53-deficient cells impaired tumor initiation and maintenance in vitro and in vivo. Mechanistic studies reveal that translation-associated pathways were enriched for Prmt1 downstream targets, implicating Prmt1 in translation control. In particular, loss of Prmt1 led to a decrease in arginine methylation of the translation initiation complex, thereby disrupting its assembly and inhibiting translation. p53/Rb-null cells were sensitive to p53-induced translation stress, and analysis of human cancer cell line data from Project Achilles further revealed that Prmt1 and translation-associated pathways converged on the same functional networks. We propose that targeted therapy against Prmt1 and its associated translation-related pathways offer a mechanistic rationale for treatment of osteosarcomas and other cancers that exhibit dependencies on translation stress response. Cancer Res; 77(17); 4613–25. ©2017 AACR.

GSEA analysis of differentially expressed genes in the control versus Prmt1 KO mOS at the transcript level using GO gene sets.

Supplementary proteomics method

GSEA analysis of gene dependencies that correlate with Prmt1 dependency in the human Project Achilles dataset.

Parallel reaction monitoring (PRM) has emerged as a popular approach for targeted protein quantification. With high ion utilization efficiency and first-in-class acquisition speed, the timsTOF Pro provides a powerful platform for PRM analysis. However, sporadic chromatographic drift in peptide retention time represents a fundamental limitation for the reproducible multiplexing of targets across PRM acquisitions. Here, we present PRM-LIVE, an extensible, Python-based acquisition engine for the timsTOF Pro, which dynamically adjusts detection windows for reproducible target scheduling. In this initial implementation, we used iRT peptides as retention time standards and demonstrated reproducible detection and quantification of 1857 tryptic peptides from the cell lysate in a 60 min PRM-LIVE acquisition. As an application in functional proteomics, we use PRM-LIVE in an activity-based protein profiling platform to assess binding selectivity of small-molecule inhibitors against 220 endogenous human kinases.

DNA double-strand breaks (DSBs) are repaired through homology-directed repair (HDR) or non-homologous end joining (NHEJ). BRCA1/2-deficient cancer cells cannot perform HDR, conferring sensitivity to poly(ADP-ribose) polymerase inhibitors (PARPi). However, concomitant loss of the pro-NHEJ factors 53BP1, RIF1, REV7-Shieldin (SHLD1-3) or CST-DNA polymerase alpha (Pol-α) in BRCA1-deficient cells restores HDR and PARPi resistance. Here, we identify the TRIP13 ATPase as a negative regulator of REV7. We show that REV7 exists in active 'closed' and inactive 'open' conformations, and TRIP13 catalyses the inactivating conformational change, thereby dissociating REV7-Shieldin to promote HDR. TRIP13 similarly disassembles the REV7-REV3 translesion synthesis (TLS) complex, a component of the Fanconi anaemia pathway, inhibiting error-prone replicative lesion bypass and interstrand crosslink repair. Importantly, TRIP13 overexpression is common in BRCA1-deficient cancers, confers PARPi resistance and correlates with poor prognosis. Thus, TRIP13 emerges as an important regulator of DNA repair pathway choice-promoting HDR, while suppressing NHEJ and TLS.

Notch signaling regulates squamous cell proliferation and differentiation and is frequently disrupted in squamous cell carcinomas, in which Notch is tumor suppressive. Here, we show that conditional activation of Notch in squamous cells activates a context-specific gene expression program through lineage-specific regulatory elements. Among direct Notch target genes are multiple DNA damage response genes, includingIER5, which we show is required for Notch-induced differentiation of squamous carcinoma cells and TERT-immortalized keratinocytes.IER5is epistatic toPPP2R2A, a gene that encodes the PP2A B55α subunit, which we show interacts with IER5 in cells and in purified systems. Thus, Notch and DNA-damage response pathways converge in squamous cells on common genes that promote differentiation, which may serve to eliminate damaged cells from the proliferative pool. We further propose that crosstalk involving Notch and PP2A enables tuning and integration of Notch signaling with other pathways that regulate squamous differentiation.

Notch signaling regulates normal squamous cell proliferation and differentiation and is frequently disrupted in squamous cell carcinomas, in which Notch is a key tumor suppressive pathway. To identify the direct targets of Notch that produce these phenotypes, we introduced a conditional Notch transgene into squamous cell carcinoma cell lines, which respond to Notch activation in 2D culture and in organoid cultures by undergoing differentiation. RNA-seq and ChIP-seq analyses show that in squamous cells Notch activates a context-specific program of gene expression that depends on lineage-specific regulatory elements, most of which lie in long- range enhancers. Among the direct Notch target genes are multiple DNA damage response genes, includingIER5, which is regulated by Notch through several enhancer elements. We show thatIER5is required for Notch-induced differentiation in squamous carcinoma cells and in TERT-immortalized keratinocytes. Its function is epistatic toPPP2R2A, which encodes the B55αsubunit of PP2A, and IER5 interacts with B55αin cells and in purified systems. These results show that Notch and DNA-damage response pathways converge in squamous cells and that some components of these pathways promote differentiation, which may serve to eliminate DNA-damaged cells from the proliferative pool in squamous epithelia. Crosstalk involving Notch and PP2A may enable Notch signaling to be tuned and integrated with other pathways that regulate squamous differentiation. Our work also suggests that squamous cell carcinomas remain responsive to Notch signaling, providing a rationale for reactivation of Notch as a therapeutic strategy in these cancers.ImpactOur findings highlight context-specific crosstalk between Notch, DNA damage response genes, and PP2A, and provide a roadmap for understanding how Notch induces the differentiation of squamous cells.

The peptidyl-prolyl isomerase, Pin1, is exploited in cancer to activate oncogenes and inactivate tumor suppressors. However, despite considerable efforts, Pin1 has remained an elusive drug target. Here, we screened an electrophilic fragment library to identify covalent inhibitors targeting Pin1’s active site Cys113, leading to the development of Sulfopin, a nanomolar Pin1 inhibitor. Sulfopin is highly selective, as validated by two independent chemoproteomics methods, achieves potent cellular and in vivo target engagement and phenocopies Pin1 genetic knockout. Pin1 inhibition had only a modest effect on cancer cell line viability. Nevertheless, Sulfopin induced downregulation of c-Myc target genes, reduced tumor progression and conferred survival benefit in murine and zebrafish models of MYCN-driven neuroblastoma, and in a murine model of pancreatic cancer. Our results demonstrate that Sulfopin is a chemical probe suitable for assessment of Pin1-dependent pharmacology in cells and in vivo, and that Pin1 warrants further investigation as a potential cancer drug target.

Complex neural circuitry requires stable connections formed by lengthy axons. To maintain these functional circuits, fast transport delivers RNAs to distal axons where they undergo local translation. However, the mechanism that enables long distance transport of non-membrane enclosed organelles such as RNA granules is not known. Here we demonstrate that a complex containing RNA and the RNA-binding protein (RBP) SFPQ interacts directly with a tetrameric kinesin containing the adaptor KLC1 and the motor KIF5A. We show that binding of SFPQ to KIF5A/KLC1 motor complex is required for axon survival and is impacted by KIF5A mutations that cause Charcot-Marie-Tooth (CMT) Disease. Moreover, therapeutic approaches that bypass the need for local translation of SFPQ-bound proteins prevent axon degeneration in CMT models. Collectively, these observations show that non-membrane enclosed organelles can move autonomously and that replacing axonally translated proteins provides a therapeutic approach to axonal degenerative disorders.

Deleted in Breast Cancer 1 (DBC1) is an important metabolic sensor. Previous studies have implicated DBC1 in various cellular functions, notably cell proliferation, apoptosis, histone modification, and adipogenesis. However, current reports about the role of DBC1 in tumorigenesis are controversial and designate DBC1 alternatively as a tumor suppressor or a tumor promoter. In the present study, we report that polyoma small T antigen (PyST) associates with DBC1 in mammalian cells, and this interaction leads to the posttranslational downregulation of DBC1 protein levels. When coexpressed, DBC1 overcomes PyST-induced mitotic arrest and promotes the exit of cells from mitosis. Using both transient and stable modes of PyST expression, we also show that cellular DBC1 is subjected to degradation by LKB1, a tumor suppressor and cellular energy sensor kinase, in an AMP kinase-independent manner. Moreover, LKB1 negatively regulates the phosphorylation as well as activity of the prosurvival kinase AKT1 through DBC1 and its downstream pseudokinase substrate, Tribbles 3 (TRB3). Using both transient transfection and stable cell line approaches as well as soft agar assay, we demonstrate that DBC1 has oncogenic potential. In conclusion, our study provides insight into a novel signaling axis that connects LKB1, DBC1, TRB3, and AKT1. We propose that the LKB1–DBC1–AKT1 signaling paradigm may have an important role in the regulation of cell cycle and apoptosis and consequently tumorigenesis.

Through an shRNA screen, we identified the protein arginine methyltransferase Prmt1 as a vulnerable intervention point in murine p53/Rb-null osteosarcomas, the human counterpart of which lacks effective therapeutic options. Depletion of Prmt1 in p53-deficient cells impaired tumor initiation and maintenance in vitro and in vivo. Mechanistic studies reveal that translation-associated pathways were enriched for Prmt1 downstream targets, implicating Prmt1 in translation control. In particular, loss of Prmt1 led to a decrease in arginine methylation of the translation initiation complex, thereby disrupting its assembly and inhibiting translation. p53/Rb-null cells were sensitive to p53-induced translation stress, and analysis of human cancer cell line data from Project Achilles further revealed that Prmt1 and translation-associated pathways converged on the same functional networks. We propose that targeted therapy against Prmt1 and its associated translation-related pathways offer a mechanistic rationale for treatment of osteosarcomas and other cancers that exhibit dependencies on translation stress response. Cancer Res; 77(17); 4613–25. ©2017 AACR.

Alterations involving serine-threonine phosphatase PP2A subunits occur in a range of human cancers, and partial loss of PP2A function contributes to cell transformation. Displacement of regulatory B subunits by the SV40 Small T antigen (ST) or mutation/deletion of PP2A subunits alters the abundance and types of PP2A complexes in cells, leading to transformation. Here, we show that ST not only displaces common PP2A B subunits but also promotes A-C subunit interactions with alternative B subunits (B’’’, striatins) that are components of the Striatin-interacting phosphatase and kinase (STRIPAK) complex. We found that STRN4, a member of STRIPAK, is associated with ST and is required for ST-PP2A-induced cell transformation. ST recruitment of STRIPAK facilitates PP2A-mediated dephosphorylation of MAP4K4 and induces cell transformation through the activation of the Hippo pathway effector YAP1. These observations identify an unanticipated role of MAP4K4 in transformation and show that the STRIPAK complex regulates PP2A specificity and activity., eLife digest Cells maintain a fine balance of signals that promote or counter cell growth and division. Two sets of enzymes – called kinases and phosphatases – contribute to this balance. In general, kinases “switch on” other proteins by tagging them with a phosphate molecule. This process is called phosphorylation. Phosphatases, on the other hand, dephosphorylate these proteins, switching them off. Cancer cells often have mutations that activate kinases to drive cancer growth. The same cells can have mutations that inactivate the phosphatases or reduce their abundance. The roles of phosphatases in cancer are still being studied. One major hurdle in this research is that it is not always clear how they recognize the proteins they dephosphorylate. Protein phosphatase 2A (or PP2A for short) is one of the phosphatases that is often mutated or deleted in human cancers. Even just reduced levels of PP2A can promote cancer. Kim, Berrios, Kim, Schade et al. used an experimental trick to decrease the phosphatase activity of PP2A in human cells growing in a dish. Biochemical analysis of these cells showed that, as expected, many proteins were now in their phosphorylated states. Unexpectedly, however, some proteins were dephosphorylated under these conditions. One of these proteins was called MAP4K4. In the case of MAP4K4, the dephosphorylated state contributes to the growth of the cancer cell. Kim et al. carried out further genetic and biochemical experiments to show that, in these cells, PP2A and MAP4K4 stay physically connected to one another. This connection was enabled by a group of proteins called the STRIPAK complex. The STRIPAK proteins directed the remaining PP2A towards MAP4K4. Low levels or activity of PP2A could, therefore, promote cancer in a different way. Taken together, PP2A is not a single phosphatase that always turns proteins off, but rather is a dual switch that turns off some proteins while turning on others. Future experiments will explore to what extent these findings also apply in tumors. Information about how mutations in PP2A affect human cancers could suggest new targets for cancer drugs.

Alterations involving serine-threonine phosphatase PP2A subunits occur in a range of human cancers and partial loss of PP2A function contributes to cell transformation. Displacement of regulatory B subunits by the SV40 Small T antigen (ST) or mutation/deletion of PP2A subunits alters the abundance and types of PP2A complexes in cells, leading to transformation. Here we show that ST not only displaces common PP2A B subunits but also promotes A-C subunit interactions with alternative B subunits (B’’’, striatins) that are components of the Striatin-interacting phosphatase and kinase (STRIPAK) complex. We found that STRN4, a member of STRIPAK, is associated with ST and is required for ST-PP2A-induced cell transformation. ST recruitment of STRIPAK facilitates PP2A-mediated dephosphorylation of MAP4K4 and induces cell transformation through the activation of the Hippo pathway effector YAP1. These observations identify an unanticipated role of MAP4K4 in transformation and show that the STRIPAK complex regulates PP2A specificity and activity.

DBC1 plays a critical role in various cellular functions notably cell proliferation, transcription, histone modification and adipogenesis. Current reports about the role of DBC1 in tumorigenesis are paradoxical and designate DBC1 both as a tumor suppressor or an oncogene. Here, using small T antigen of polyoma virus (PyST) as a tool, we have delineated a signaling mechanism that connects LKB1 to AKT1 via DBC1. We report that PyST associates with DBC1 and leads to its down-regulation. Our results also show that PyST expression promotes LKB1 activation which in turn leads to in the downregulation of DBC1 protein. Absence of DBC1 results in transcriptional upregulation and consequently enhanced protein levels of TRB3. TRB3 sequesters AKT1, and consequently the phosphorylation and activity of AKT1 is compromised. This ultimately results in inactivation of pro-survival pathways triggered via AKT1 signaling. Our studies thus provide an insight into a signaling pathway that connects LKB1, DBC1, TRB3 and AKT1.

Affinity purification followed by mass spectrometry has become the technique of choice to identify binding partners in biochemical complexes isolated from a physiologic cellular context. In this report we detail our protocol for tandem affinity purification (TAP) primarily based on the use of the FLAG and HA peptide epitopes, with a particular emphasis on factors affecting yield and specificity, as well as steps to implement an automated version of the TAP procedure. © 2019 by John Wiley & Sons, Inc.

COP1 proteins are E3 ubiquitin ligases that regulate phototropism in plants and target transcription factors for degradation in mammals. The substrate-binding region of COP1 resides within a WD40-repeat domain that also binds to Trib proteins, which are adaptors for C/EBPα degradation. We report here structures of the human COP1 WD40 domain in isolation, and complexes of the human and Arabidopsis thaliana COP1 WD40 domains with the binding motif of Trib1. The human and Arabidopsis WD40 domains are seven-bladed β-propellers with an inserted loop on the bottom face of the first blade. The Trib1 peptide binds in an extended conformation to a highly conserved surface on the top face of the β-propeller, indicating a general mode for recognition of peptide motifs by COP1. Together, these studies identify the structural basis and key interactions for motif recognition by COP1, and hint at how Trib1 autoinhibition is overcome to target C/EBPα for degradation.

Comment in :"DNA Methylation and Chromatin: Role(s) of Methyl-CpG-Binding Protein ZBTB38. [Epigenet Insights. 2018]"; International audience; Reactive oxygen species (ROS) are a byproduct of cell metabolism, and can also arise from environmental sources, such as toxins or radiation. Depending on dose and context, ROS have both beneficial and deleterious roles in mammalian development and disease, therefore it is crucial to understand how these molecules are generated, sensed, and detoxified. The question of how oxidative stress connects to the epigenome, in particular, is important yet incompletely understood. Here we show that an epigenetic regulator, the methyl-CpG-binding protein ZBTB38, limits the basal cellular production of ROS, is induced by ROS, and is required to mount a proper response to oxidative stress. Molecularly, these functions depend on a deubiquitinase, USP9X, which interacts with ZBTB38, deubiquitinates it, and stabilizes it. We find that USP9X is itself stabilized by oxidative stress, and is required together with ZBTB38 to limit the basal generation of ROS, as well as the toxicity of an acute oxidative stress. Our data uncover a new nuclear target of USP9X, show that the USP9X/ZBTB38 axis limits, senses and detoxifies ROS, and provide a molecular link between oxidative stress and the epigenome.

Targeting the activation function-1 (AF-1) domain located in the N-terminus of the androgen receptor (AR) is an attractive therapeutic alternative to the current approaches to inhibit AR action in prostate cancer (PCa). Here we show that the AR AF-1 is bound by the cochaperone Bag-1L. Mutations in the AR interaction domain or loss of Bag-1L abrogate AR signaling and reduce PCa growth. Clinically, Bag-1L protein levels increase with progression to castration-resistant PCa (CRPC) and high levels of Bag-1L in primary PCa associate with a reduced clinical benefit from abiraterone when these tumors progress. Intriguingly, residues in Bag-1L important for its interaction with the AR AF-1 are within a potentially druggable pocket, implicating Bag-1L as a potential therapeutic target in PCa., eLife digest Prostate cancer is the second most common cancer in men around the world. The cancer relies on a protein called the androgen receptor in order to develop and grow. Currently, some of the most common treatments for prostate cancer, especially in its advanced stages, are drugs that block the activity of this receptor. However, such treatments are only successful for a limited period of time, and so alternative methods to inhibit this receptor are still needed. The androgen receptor must bind to a number of proteins to carry out its activity. These proteins include one called Bag-1L, which is also important for the development of prostate cancer. Stopping such a protein from binding with the androgen receptor might represent a new way to treat prostate cancer; but first it will be important to understand how this interaction actually regulates the activity of the receptor. Now, Cato et al. have analyzed samples of cancer cells that had been collected from 43 patients with prostate cancer and found that Bag-1L levels increase as the disease progresses. Looking at the patients’ medical records then revealed that therapies targeting the androgen receptor were less effective in people with high levels of Bag-1L. Conversely, altering, removing or inhibiting Bag-1L in prostate cancer cells grown in the laboratory made the receptor less active and made the cells grow slower. Further experiments went on to reveal that Bag-1L interacts with a regulatory region of the androgen receptor. Cato et al. note that this region remains largely unexplored therapeutically, because it has some unique structural properties that restrict how much it can interact with drug molecules. Targeting Bag-1L and stopping it from binding to this region of the androgen receptor would represent a different approach to inhibiting the androgen receptor and treating patients with prostate cancer. Together these new findings should provide pharmaceutical companies with much of the information they would require to immediately start screening for therapies that target Bag-1L. Ultimately, Cato et al. hope that any follow-up findings will benefit prostate cancer patients by improving the currently available treatments.

To maintain cellular homeostasis, subcellular organelles communicate with each other and form physical and functional networks through membrane contact sites coupled by protein tethers. In particular, endoplasmic reticulum (ER)–mitochondrial contacts (EMC) regulate diverse cellular activities such as metabolite exchange (Ca2+ and lipids), intracellular signaling, apoptosis, and autophagy. The significance of EMCs has been highlighted by reports indicating that EMC dysregulation is linked to neurodegenerative diseases. Therefore, obtaining a better understanding of the physical and functional components of EMCs should provide new insights into the pathogenesis of several neurodegenerative diseases. Here, we applied engineered ascorbate peroxidase (APEX) to map the proteome at EMCs in live HEK293 cells. APEX was targeted to the outer mitochondrial membrane, and proximity-labeled proteins were analyzed by stable isotope labeling with amino acids in culture (SILAC)-LC/MS-MS. We further refined the specificity of the proteins identified by combining biochemical subcellular fractionation to the protein isolation method. We identified 405 proteins with a 2.0-fold cutoff ratio (log base 2) in SILAC quantification from replicate experiments. We performed validation screening with a Split-Rluc8 complementation assay that identified reticulon 1A (RTN1A), an ER-shaping protein localized to EMCs as an EMC promoter. Proximity mapping augmented with biochemical fractionation and additional validation methods reported here could be useful to discover other components of EMCs, identify mitochondrial contacts with other organelles, and further unravel their communication.

The p110β isoform of PI3K is preferentially activated in many tumors deficient in the phosphatase and tensin homolog (PTEN). However, the mechanism(s) linking PTEN loss to p110β activation remain(s) mysterious. Here, we identify CRKL as a member of the class of PI3Kβ-interacting proteins. Silencing CRKL expression in PTEN-null human cancer cells leads to a decrease in p110β-dependent PI3K signaling and cell proliferation. In contrast, CRKL depletion does not impair p110α-mediated signaling. Further study showed that CRKL binds to tyrosine-phosphorylated p130Cas in PTEN-null cancer cells. Since Src family kinases are known both to be regulated by PTEN and to phosphorylate and activate p130Cas, we tested and found that Src inhibition cooperated with p110β inhibition to suppress the growth of PTEN-null cells. These data suggest both a potential mechanism linking PTEN loss to p110β activation and the possible benefit of dual inhibition of Src and PI3K for PTEN-null tumors.

The continued evolution of modern mass spectrometry instrumentation and associated methods represents a critical component in efforts to decipher the molecular mechanisms which underlie normal physiology and understand how dysregulation of biological pathways contributes to human disease. The increasing scale of these experiments combined with the technological diversity of mass spectrometers presents several challenges for community-wide data access, analysis, and distribution. Here we detail a redesigned version of multiplierz, our Python software library which leverages our common application programming interface (mzAPI) for analysis and distribution of proteomic data. New features include support for a wider range of native mass spectrometry file types, interfaces to additional database search engines, compatibility with new reporting formats, and high-level tools to perform post-search proteomic analyses. A GUI desktop environment, mzDesktop, provides access to multiplierz functionality through a user friendly interface. multiplierz is available for download from: https://github.com/BlaisProteomics/multiplierz; and mzDesktop is available for download from: https://sourceforge.net/projects/multiplierz/.

Activating mutations in genes encoding G protein α (Gα) subunits occur in 4-5% of all human cancers, but oncogenic alterations in Gβ subunits have not been defined. Here we demonstrate that recurrent mutations in the Gβ proteins GNB1 and GNB2 confer cytokine-independent growth and activate canonical G protein signaling. Multiple mutations in GNB1 affect the protein interface that binds Gα subunits as well as downstream effectors and disrupt Gα interactions with the Gβγ dimer. Different mutations in Gβ proteins clustered partly on the basis of lineage; for example, all 11 GNB1 K57 mutations were in myeloid neoplasms, and seven of eight GNB1 I80 mutations were in B cell neoplasms. Expression of patient-derived GNB1 variants in Cdkn2a-deficient mouse bone marrow followed by transplantation resulted in either myeloid or B cell malignancies. In vivo treatment with the dual PI3K-mTOR inhibitor BEZ235 suppressed GNB1-induced signaling and markedly increased survival. In several human tumors, mutations in the gene encoding GNB1 co-occurred with oncogenic kinase alterations, including the BCR-ABL fusion protein, the V617F substitution in JAK2 and the V600K substitution in BRAF. Coexpression of patient-derived GNB1 variants with these mutant kinases resulted in inhibitor resistance in each context. Thus, GNB1 and GNB2 alterations confer transformed and resistance phenotypes across a range of human tumors and may be targetable with inhibitors of G protein signaling.

Cell migration is essential for embryonic development and tissue formation in all animals. cacn-1 is a conserved gene of unknown molecular function identified in a genome-wide screen for genes that regulate distal tip cell migration in the nematode worm Caenorhabditis elegans. In this study we take a proteomics approach to understand CACN-1 function. To isolate CACN-1−interacting proteins, we used an in vivo tandem-affinity purification strategy. Tandem-affinity purification−tagged CACN-1 complexes were isolated from C. elegans lysate, analyzed by mass spectrometry, and characterized bioinformatically. Results suggest significant interaction of CACN-1 with the C. elegans spliceosome. All of the identified interactors were screened for distal tip cell migration phenotypes using RNAi. Depletion of many of these factors led to distal tip cell migration defects, particularly a failure to stop migrating, a phenotype commonly seen in cacn-1 deficient animals. The results of this screen identify eight novel regulators of cell migration and suggest CACN-1 may participate in a protein network dedicated to high-fidelity gonad development. The composition of proteins comprising the CACN-1 network suggests that this critical developmental module may exert its influence through alternative splicing or other post-transcriptional gene regulation.

The bHLH transcription factor Olig2 is expressed in cycling neural progenitor cells but also in terminally differentiated, myelinating oligodendrocytes. Sustained expression of Olig2 is counterintuitive because all known functions of the protein in expansion of neural progenitors and specification of oligodendrocyte progenitors are completed with the formation of mature white matter. How are the biological functions of Olig2 suppressed in terminally differentiated oligodendrocytes? In previous studies, we have shown that a triple serine motif in the amino terminus of Olig2 is phosphorylated in cycling neural progenitors but not in their differentiated progeny. We now show that phosphorylation of the triple serine motif regulates intranuclear compartmentalization of murine Olig2. Phosphorylated Olig2 is preferentially localized to a transcriptionally active "open" chromatin compartment together with coregulator proteins essential for regulation of gene expression. Unphosphorylated Olig2, as seen in mature white matter, is localized mainly within a transcriptionally inactive, chromatin fraction characterized by condensed and inaccessible DNA. Of special note is the observation that the p53 tumor suppressor protein is confined to the open chromatin fraction. Proximity ligation assays show that phosphorylation brings Olig2 within 30 nm of p53 within the open chromatin compartment. The data thus shed light on previously noted promitogenic functions of phosphorylated Olig2, which reflect, at least in part, an oppositional relationship with p53 functions.

The transcription factor SOX2 is an essential regulator of pluripotent stem cells and promotes development and maintenance of squamous epithelia. We previously reported that SOX2 is an oncogene and subject to highly recurrent genomic amplification in squamous cell carcinomas (SCCs). Here, we have further characterized the function of SOX2 in SCC. Using ChIP-seq analysis, we compared SOX2-regulated gene profiles in multiple SCC cell lines to ES cell profiles and determined that SOX2 binds to distinct genomic loci in SCCs. In SCCs, SOX2 preferentially interacts with the transcription factor p63, as opposed to the transcription factor OCT4, which is the preferred SOX2 binding partner in ES cells. SOX2 and p63 exhibited overlapping genomic occupancy at a large number of loci in SCCs; however, coordinate binding of SOX2 and p63 was absent in ES cells. We further demonstrated that SOX2 and p63 jointly regulate gene expression, including the oncogene ETV4, which was essential for SOX2-amplified SCC cell survival. Together, these findings demonstrate that the action of SOX2 in SCC differs substantially from its role in pluripotency. The identification of the SCC-associated interaction between SOX2 and p63 will enable deeper characterization the downstream targets of this interaction in SCC and normal squamous epithelial physiology.

The Ku heterodimer plays an essential role in non-homologous end-joining and other cellular processes including transcription, telomere maintenance and apoptosis. While the function of Ku is regulated through its association with other proteins and nucleic acids, the specific composition of these macromolecular complexes and their dynamic response to endogenous and exogenous cellular stimuli are not well understood. Here we use quantitative proteomics to define the composition of Ku multicomponent complexes and demonstrate that they are dramatically altered in response to UV radiation. Subsequent biochemical assays revealed that the presence of DNA ends leads to the substitution of RNA-binding proteins with DNA and chromatin associated factors to create a macromolecular complex poised for DNA repair. We observed that dynamic remodeling of the Ku complex coincided with exit of Ku and other DNA repair proteins from the nucleolus. Microinjection of sheared DNA into live cells as a mimetic for double strand breaks confirmed these findings in vivo.

Protein phosphatase PP4C has been implicated in the DNA damage response (DDR), but its substrates in DDR remain largely unknown. We devised a novel proteomic strategy for systematic identification of proteins dephosphorylated by PP4C and identified KRAB-domain-associated protein 1 (KAP-1) as a substrate. Ionizing radiation leads to phosphorylation of KAP-1 at S824 (via ATM) and at S473 (via CHK2). A PP4C/R3β complex interacts with KAP-1 and silencing this complex leads to persistence of phospho-S824 and phospho-S473. We identify a new role for KAP-1 in DDR by showing that phosphorylation of S473 impacts the G2/M checkpoint. Depletion of PP4R3β or expression of the phosphomimetic KAP-1 S473 mutant (S473D) leads to a prolonged G2/M checkpoint. Phosphorylation of S824 is necessary for repair of heterochromatic DNA lesions and similar to cells expressing phosphomimetic KAP-1 S824 mutant (S824D), or PP4R3β-silenced cells, display prolonged relaxation of chromatin with release of chromatin remodelling protein CHD3. Our results define a new role for PP4-mediated dephosphorylation in the DDR, including the regulation of a previously undescribed function of KAP-1 in checkpoint response.

BRCA1 contributes to the response to UV irradiation. Utilizing its BRCT motifs, it is recruited during S/G2 to UV-damaged sites in a DNA replication-dependent but nucleotide excision repair (NER)-independent manner. More specifically, at UV-stalled replication forks, it promotes photoproduct excision, suppression of translesion synthesis, and the localization and activation of replication factor C complex (RFC) subunits. The last function, in turn, triggers post-UV checkpoint activation and postreplicative repair. These BRCA1 functions differ from those required for DSBR.

Genetic rearrangements of the anaplastic lymphoma kinase (ALK) kinase occur in 3% to 13% of non–small cell lung cancer patients and rarely coexist with KRASor EGFR mutations. To evaluate potential treatment strategies for lung cancers driven by an activated EML4-ALK chimeric oncogene, we generated a genetically engineered mouse model that phenocopies the human disease where this rearranged gene arises. In this model, the ALK kinase inhibitor TAE684 produced greater tumor regression and improved overall survival compared with carboplatin and paclitaxel, representing clinical standard of care. 18F-FDG-PET-CT scans revealed almost complete inhibition of tumor metabolic activity within 24 hours of TAE684 exposure. In contrast, combined inhibition of the PI3K/AKT and MEK/ERK1/2 pathways did not result in significant tumor regression. We identified EML4-ALK in complex with multiple cellular chaperones including HSP90. In support of a functional reliance, treatment with geldanamycin-based HSP90 inhibitors resulted in rapid degradation of EML4-ALK in vitro and substantial, albeit transient, tumor regression in vivo. Taken together, our findings define a murine model that offers a reliable platform for the preclinical comparison of combinatorial treatment approaches for lung cancer characterized by ALK rearrangement.Cancer Res; 70(23); 9827–36. ©2010 AACR.

Tandem affinity purification (TAP) coupled with mass spectrometry has become the technique of choice for characterization of multi-component protein complexes. While current TAP protocols routinely provide high yield and specificity for proteins expressed under physiologically relevant conditions, analytical figures of merit required for efficient and in-depth LC-MS analysis remain unresolved. Here we implement a multidimensional chromatography platform, based on two stages of reversed-phase separation operated at high and low pH, respectively. We compare performance metrics for RP-RP and SCX-RP for the analysis of complex peptide mixtures derived from cell lysate, as well as protein complexes purified via TAP. Our data reveal that RP-RP fractionation outperforms SCX-RP primarily due to increased peak capacity in the first dimension separation. We integrate this system with miniaturized LC assemblies to achieve true online fractionation at low (≤5nL/min) effluent flow rates. Stable isotope labeling is used to monitor the dynamics of the multi-component Ku protein complex in response to DNA damage induced by gamma radiation.

Fas-activated serine/threonine phosphoprotein (FAST) is the founding member of the FAST kinase domain-containing protein (FASTKD) family that includes FASTKD1-5. FAST is a sensor of mitochondrial stress that modulates protein translation to promote the survival of cells exposed to adverse conditions. Mutations in FASTKD2 have been linked to a mitochondrial encephalomyopathy that is associated with reduced cytochrome c oxidase activity, an essential component of the mitochondrial electron transport chain. We have confirmed the mitochondrial localization of FASTKD2 and shown that all FASTKD family members are found in mitochondria. Although human and mouse FASTKD1-5 genes are expressed ubiquitously, some of them are most abundantly expressed in mitochondria-enriched tissues. We have found that RNA interference-mediated knockdown of FASTKD3 severely blunts basal and stress-induced mitochondrial oxygen consumption without disrupting the assembly of respiratory chain complexes. Tandem affinity purification reveals that FASTKD3 interacts with components of mitochondrial respiratory and translation machineries. Our results introduce FASTKD3 as an essential component of mitochondrial respiration that may modulate energy balance in cells exposed to adverse conditions by functionally coupling mitochondrial protein synthesis to respiration.

EP300-interacting inhibitor of differentiation 1 (EID1) belongs to a protein family implicated in the control of transcription, differentiation, DNA repair, and chromosomal maintenance. EID1 has a very short half-life, especially in G0 cells. We discovered that EID1 contains a peptidic, modular degron that is necessary and sufficient for its polyubiquitylation and proteasomal degradation. We found that this degron is recognized by an Skp1, Cullin, and F-box (SCF)-containing ubiquitin ligase complex that uses the F-box Only Protein 21 (FBXO21) as its substrate recognition subunit. SCF(FBXO21) polyubiquitylates EID1 both in vitro and in vivo and is required for the efficient degradation of EID1 in both cycling and quiescent cells. The EID1 degron partially overlaps with its retinoblastoma tumor suppressor protein-binding domain and is congruent with a previously defined melanoma-associated antigen-binding motif shared by EID family members, suggesting that binding to retinoblastoma tumor suppressor and melanoma-associated antigen family proteins could affect the polyubiquitylation and turnover of EID family members in cells.