The Cas9/CRISPR system has become a popular choice for genome editing. In this system, binding of a single guide (sg) RNA to a cognate genomic sequence enables the Cas9 nuclease to induce a double-strand break at that locus. This break is next repaired by an error-prone mechanism, leading to mutation and gene disruption. In this study we describe a range of refinements of the method, including stable cell lines expressing Cas9, and a PCR based protocol for the generation of the sgRNA. We also describe a simple methodology that allows both elimination of Cas9 from cells after gene disruption and re-introduction of the disrupted gene. This advance enables easy assessment of the off target effects associated with gene disruption, as well as phenotype-based structure-function analysis. In our study, we used the Fan1 DNA repair gene as control in these experiments. Cas9/CRISPR-mediated Fan1 disruption occurred at frequencies of around 29%, and resulted in the anticipated spectrum of genotoxin hypersensitivity, which was rescued by re-introduction of Fan1.
The roles of IL-1R-associated kinase (IRAK)2 and IRAK1 in cytokine production were investigated using immune cells from knock-in mice expressing the TNFR-associated factor 6 (TRAF6) binding-defective mutant IRAK2[E525A] or the catalytically inactive IRAK1[D359A] mutant. In bone marrow-derived macrophages (BMDMs), the IRAK2-TRAF6 interaction was required for the late (2-8 h) but not the early phase (0-2 h) of il6, and tnfa mRNA production and hence for IL-6 and TNF-α secretion by TLR agonists that signal via MyD88. Loss of the IRAK2-TRAF6 interaction had little effect on the MyD88-dependent production of anti-inflammatory molecules produced during the early phase, such as Dual Specificity Phosphatase 1, and a modest effect on IL-10 secretion. The LPS/TLR4-stimulated production of il6 and tnfa mRNA and IL-6 and TNF-α secretion was hardly affected, because the Toll/IL-1R domain-containing adapter-inducing IFN-β (TRIF) signaling pathway was used instead of the IRAK2-TRAF6 interaction to sustain late-phase mRNA production. IRAK1 catalytic activity was not rate-limiting for il6, tnfa or il10 mRNA production or the secretion of these cytokines by BMDMs, but IFN-β mRNA induction by TLR7 and TLR9 agonists was greatly delayed in plasmacytoid dendritic cells (pDCs) from IRAK1[D359A] mice. In contrast, IFN-β mRNA production was little affected in pDCs from IRAK2[E525A] mice, but subsequent IFN-α mRNA production and IFN-α secretion were reduced. IFN-β and IFN-α production were abolished in pDCs from IRAK1[D359A]×IRAK2[E525A] double knock-in mice. Our results establish that the IRAK2-TRAF6 interaction is rate limiting for the late, but not the early phase of cytokine production in BMDM and pDCs, and that the IRAK2-TRAF6 interaction is needed to sustain IκB-inducing kinase β activity during prolonged activation of the MyD88 signalling network.
The dimeric 14-3-3 proteins dock onto pairs of phosphorylated Ser and Thr residues on hundreds of proteins, and thereby regulate many events in mammalian cells. To facilitate global analyses of these interactions, we developed a web resource named ANIA: ANnotation and Integrated Analysis of the 14-3-3 interactome, which integrates multiple data sets on 14-3-3-binding phosphoproteins. ANIA also pinpoints candidate 14-3-3-binding phosphosites using predictor algorithms, assisted by our recent discovery that the human 14-3-3-interactome is highly enriched in 2R-ohnologues. 2R-ohnologues are proteins in families of two to four, generated by two rounds of whole genome duplication at the origin of the vertebrate animals. ANIA identifies candidate ‘lynchpins’, which are 14-3-3-binding phosphosites that are conserved across members of a given 2R-ohnologue protein family. Other features of ANIA include a link to the catalogue of somatic mutations in cancer database to find cancer polymorphisms that map to 14-3-3-binding phosphosites, which would be expected to interfere with 14-3-3 interactions. We used ANIA to map known and candidate 14-3-3-binding enzymes within the 2R-ohnologue complement of the human kinome. Our projections indicate that 14-3-3s dock onto many more human kinases than has been realized. Guided by ANIA, PAK4, 6 and 7 (p21-activated kinases 4, 6 and 7) were experimentally validated as a 2R-ohnologue family of 14-3-3-binding phosphoproteins. PAK4 binding to 14-3-3 is stimulated by phorbol ester, and involves the ‘lynchpin’ site phosphoSer99 and a major contribution from Ser181. In contrast, PAK6 and PAK7 display strong phorbol ester-independent binding to 14-3-3, with Ser113 critical for the interaction with PAK6. These data point to differential 14-3-3 regulation of PAKs in control of cell morphology.
Database URL: https://ania-1433.lifesci.dundee.ac.uk/prediction/webserver/index.py
Loss of iron triggers PINK1/Parkin-independent mitophagy
A novel mitophagy assay uncovers a new PINK1/Parkin-independent mitophagy pathway induced by a decrease in iron levels. This pathway is active in fibroblasts of Parkinson patients with Parkin mutations and could be exploited as a potential therapy.
In this study, we develop a simple assay to identify mitophagy inducers on the basis of the use of fluorescently tagged mitochondria that undergo a colour change on lysosomal delivery. Using this assay, we identify iron chelators as a family of compounds that generate a strong mitophagy response. Iron chelation-induced mitophagy requires that cells undergo glycolysis, but does not require PINK1 stabilization or Parkin activation, and occurs in primary human fibroblasts as well as those isolated from a Parkinson's patient with Parkin mutations. Thus, we have identified and characterized a mitophagy pathway, the induction of which could prove beneficial as a potential therapy for several neurodegenerative diseases in which mitochondrial clearance is advantageous.
autophagy; iron/mitophagy; PINK1; Parkin
Holliday junctions (HJs) are X-shaped DNA structures that arise during homologous recombination, which must be removed to enable chromosome segregation. The SLX1 and MUS81-EME1 nucleases can both process HJs in vitro, and they bind in close proximity on the SLX4 scaffold, hinting at possible cooperation. However, the cellular roles of mammalian SLX1 are not yet known. Here, we use mouse genetics and structure function analysis to investigate SLX1 function. Disrupting the murine Slx1 and Slx4 genes revealed that they are essential for HJ resolution in mitotic cells. Moreover, SLX1 and MUS81-EME1 act together to resolve HJs in a manner that requires tethering to SLX4. We also show that SLX1, like MUS81-EME1, is required for repair of DNA interstrand crosslinks, but this role appears to be independent of HJ cleavage, at least in mouse cells. These findings shed light on HJ resolution in mammals and on maintenance of genome stability.
•Resolution of Holliday junctions in mouse cells requires the SLX1 nuclease•SLX1 acts cooperatively with MUS81-EME1 in HJ resolution and ICL repair•Mutations in SLX4 that prevent it binding to SLX1 and MUS81-EME1 abolish HJ resolution•DNA substrates of SLX1 and MUS81-EME1 in ICL repair appear to be different from HJs
SLX4, a scaffold for structure-specific DNA repair nucleases, is important for several types of DNA repair. Many repair proteins bind to sites of DNA damage, resulting in subnuclear “foci,” but SLX4 forms foci in human cells even without DNA damage. Using several approaches, we show that most, but not all, SLX4 foci localize to telomeres in a range of human cell lines irrespective of the mechanisms used to maintain telomere length. The SLX1 Holliday-junction-processing enzyme is recruited to telomeres by SLX4, and SLX4, in turn, is recruited by a motif that binds to the shelterin subunit TRF2 directly. We also show that TRF2-dependent recruitment of SLX4 prevents telomere damage. Furthermore, SLX4 prevents telomere lengthening and fragility in a manner that appears to be independent of telomere association. These findings reveal that SLX4 plays multiple roles in regulating telomere homeostasis.
•The SLX4 DNA repair complex binds to telomeres in different human cell lines•The SLX4 scaffold protein has a TRF2-binding motif responsible for telomere localization•SLX4 also has localization-independent roles at telomeres
SLX4 is a scaffold protein that regulates several DNA repair endonucleases and forms subnuclear foci in human cells that Rouse and colleagues identify as telomeres. SLX4 is recruited to telomeres by a motif that interacts with the shelterin subunit TRF2. Mutations in this motif prevent SLX4 from localizing at telomeres and cause telomeric DNA damage in human cells. SLX4 also has localization-independent roles at telomeres, such as preventing telomere overlengthening.
Phosphoinositide-dependent kinase-1 (PDK1) controls the activation of a subset of AGC kinases. Using a conditional knockout of PDK1 in haematopoietic cells, we demonstrate that PDK1 is essential for B cell development. B-cell progenitors lacking PDK1 arrested at the transition of pro-B to pre-B cells, due to a cell autonomous defect. Loss of PDK1 decreased the expression of the IgH chain in pro-B cells due to impaired recombination of the IgH distal variable segments, a process coordinated by the transcription factor Pax5. The expression of Pax5 in pre-B cells was decreased in PDK1 knockouts, which correlated with reduced expression of the Pax5 target genes IRF4, IRF8 and Aiolos. As a result, Ccnd3 is upregulated in PDK1 knockout pre-B cells and they have an impaired ability to undergo cell-cycle arrest, a necessary event for Ig light chain rearrangement. Instead, these cells underwent apoptosis that correlated with diminished expression of the pro-survival gene Bcl2A1. Reintroduction of both Pax5 and Bcl2A1 together into PDK1 knockout pro-B cells restored their ability to differentiate in vitro into mature B cells.
PDK1 regulates VDJ recombination, cell-cycle exit and survival during B-cell development
The conditional knockout of PDK1 in haematopoietic cells reveals an essential role in cell-cycle arrest, differentiation, IgH recombination and the survival of B cells, which is mediated by the pro-survival factor Bcl2A1 and the transcription factor Pax5.
B cell; Pax5; PDK1; preB; VDJ recombination
The polarization of macrophages into a regulatory-like phenotype and the production of IL-10 plays an important role in the resolution of inflammation. We show in this study that PGE2, in combination with LPS, is able to promote an anti-inflammatory phenotype in macrophages characterized by high expression of IL-10 and the regulatory markers SPHK1 and LIGHT via a protein kinase A–dependent pathway. Both TLR agonists and PGE2 promote the phosphorylation of the transcription factor CREB on Ser133. However, although CREB regulates IL-10 transcription, the mutation of Ser133 to Ala in the endogenous CREB gene did not prevent the ability of PGE2 to promote IL-10 transcription. Instead, we demonstrate that protein kinase A regulates the phosphorylation of salt-inducible kinase 2 on Ser343, inhibiting its ability to phosphorylate CREB-regulated transcription coactivator 3 in cells. This in turn allows CREB-regulated transcription coactivator 3 to translocate to the nucleus where it serves as a coactivator with the transcription factor CREB to induce IL-10 transcription. In line with this, we find that either genetic or pharmacological inhibition of salt-inducible kinases mimics the effect of PGE2 on IL-10 production.
Phosphorylation of insulin receptor substrate (IRS)-2 on tyrosine residues is a key event in IGF-1/insulin signaling and leads to activation of the PI 3-kinase and the Ras/MAPK pathway. Furthermore, phosphorylated serine/threonine residues on IRS-2 can induce 14-3-3 binding. In this study we searched IRS-2 for novel phosphorylation sites and investigated the interaction between IRS-2 and 14-3-3. Mass spectrometry identified a total of 24 serine/threonine residues on IRS-2 with 12 sites unique for IRS-2 while the other residues are conserved in IRS-1 and IRS-2. IGF-1 stimulation led to increased binding of 14-3-3 to IRS-2 in transfected HEK293 cells and this binding was prevented by inhibition of the PI 3-kinase pathway and an Akt/PKB inhibitor. Insulin-stimulated interaction between endogenous IRS-2 and 14-3-3 was observed in rat hepatoma cells and in mice liver after an acute insulin stimulus and refeeding. Using different IRS-2 fragments enabled localization of the IGF-1-dependent 14-3-3 binding region spanning amino acids 300–600. The 24 identified residues on IRS-2 included several 14-3-3 binding candidates in the region 300–600. Single alanine mutants of these candidates led to the identification of serine 573 as 14-3-3 binding site. A phospho-site specific antibody was generated to further characterize serine 573. IGF-1-dependent phosphorylation of serine 573 was reduced by inhibition of PI 3-kinase and Akt/PKB. A negative role of this phosphorylation site was implicated by the alanine mutant of serine 573 which led to enhanced phosphorylation of Akt/PKB in an IGF-1 time course experiment. To conclude, our data suggest a physiologically relevant role for IGF-1/insulin-dependent 14-3-3 binding to IRS-2 involving serine 573.
14-3-3 proteins regulate cellular responses to stimuli by docking onto pairs of phosphorylated residues on target proteins. The present study shows that the human 14-3-3-binding phosphoproteome is highly enriched in 2R-ohnologues, which are proteins in families of two to four members that were generated by two rounds of whole genome duplication at the origin of the vertebrates. We identify 2R-ohnologue families whose members share a ‘lynchpin’, defined as a 14-3-3-binding phosphosite that is conserved across members of a given family, and aligns with a Ser/Thr residue in pro-orthologues from the invertebrate chordates. For example, the human receptor expression enhancing protein (REEP) 1–4 family has the commonest type of lynchpin motif in current datasets, with a phosphorylatable serine in the –2 position relative to the 14-3-3-binding phosphosite. In contrast, the second 14-3-3-binding sites of REEPs 1–4 differ and are phosphorylated by different kinases, and hence the REEPs display different affinities for 14-3-3 dimers. We suggest a conceptual model for intracellular regulation involving protein families whose evolution into signal multiplexing systems was facilitated by 14-3-3 dimer binding to lynchpins, which gave freedom for other regulatory sites to evolve. While increased signalling complexity was needed for vertebrate life, these systems also generate vulnerability to genetic disorders.
Branchiostoma; Ciona; hereditary spastic paraplegia; RAB3GAP1; RAB3GAP2
The polyubiquitin-binding domain of ABIN1 limits TLR-induced MyD88 signaling to prevent spontaneous autoimmunity in mice.
The protein ABIN1 possesses a polyubiquitin-binding domain homologous to that present in nuclear factor κB (NF-κB) essential modulator (NEMO), a component of the inhibitor of NF-κB (IκB) kinase (IKK) complex. To address the physiological significance of polyubiquitin binding, we generated knockin mice expressing the ABIN1[D485N] mutant instead of the wild-type (WT) protein. These mice developed all the hallmarks of autoimmunity, including spontaneous formation of germinal centers, isotype switching, and production of autoreactive antibodies. Autoimmunity was suppressed by crossing to MyD88−/− mice, demonstrating that toll-like receptor (TLR)–MyD88 signaling pathways are needed for the phenotype to develop. The B cells and myeloid cells of the ABIN1[D485N] mice showed enhanced activation of the protein kinases TAK, IKK-α/β, c-Jun N-terminal kinases, and p38α mitogen-activated protein kinase and produced more IL-6 and IL-12 than WT. The mutant B cells also proliferated more rapidly in response to TLR ligands. Our results indicate that the interaction of ABIN1 with polyubiquitin is required to limit the activation of TLR–MyD88 pathways and prevent autoimmunity.
Although genome-wide hypomethylation is a hallmark of many cancers, roles for active DNA demethylation during tumorigenesis are unknown. Here, loss of the APC tumor suppressor gene causes upregulation of a DNA demethylase system and the concomitant hypomethylation of key intestinal cell fating genes. Notably, this hypomethylation maintained zebrafish intestinal cells in an undifferentiated state which was released upon knock down of demethylase components. Mechanistically, the demethylase genes are directly activated by Pou5f1 and Cebpβ, and indirectly repressed by retinoic acid, which antagonizes Pou5f1 and Cebpβ. Apc mutants lack retinoic acid, due to the transcriptional repression of retinol dehydrogenase l1 via a complex that includes Lef1, Groucho2, Ctbp1, Lsd1 and Corest. Our findings imply a model wherein APC controls intestinal cell fating through a switch in DNA methylation dynamics. Wildtype APC and retinoic acid downregulate demethylase components, thereby promoting DNA methylation of key genes and helping progenitors commit to differentiation.
Hundreds of candidate 14-3-3-binding (phospho)proteins have been reported in publications that describe one interaction at a time, as well as high-throughput 14-3-3-affinity and mass spectrometry-based studies. Here, we transcribed these data into a common format, deposited the collated data from low-throughput studies in MINT (http://mint.bio.uniroma2.it/mint), and compared the low- and high-throughput data in VisANT graphs that are easy to analyze and extend. Exploring the graphs prompted questions about technical and biological specificity, which were addressed experimentally, resulting in identification of phosphorylated 14-3-3-binding sites in the mitochondrial import sequence of the iron-sulfur cluster assembly enzyme (ISCU), cytoplasmic domains of the mitochondrial fission factor (MFF), and endoplasmic reticulum-tethered receptor expression-enhancing protein 4 (REEP4), RNA regulator SMAUG2, and cytoskeletal regulatory proteins, namely debrin-like protein (DBNL) and kinesin light chain (KLC) isoforms. Therefore, 14-3-3s undergo physiological interactions with proteins that are destined for diverse subcellular locations. Graphing and validating interactions underpins efforts to use 14-3-3-phosphoproteomics to identify mechanisms and biomarkers for signaling pathways in health and disease.
Compounds that inhibit signalling upstream of ERK (extracellular-signal-regulated kinase) are promising anticancer therapies, motivating research to define how this pathway promotes cancers. In the present study, we show that human capicúa represses mRNA expression for PEA3 (polyoma enhancer activator 3) Ets transcription factors ETV1, ETV4 and ETV5 (ETV is Ets translocation variant), and this repression is relieved by multisite controls of capicúa by ERK, p90RSK (p90 ribosomal S6 kinase) and 14-3-3 proteins. Specifically, 14-3-3 binds to p90RSK-phosphorylated Ser173 of capicúa thereby modulating DNA binding to its HMG (high-mobility group) box, whereas ERK phosphorylations prevent binding of a C-terminal NLS (nuclear localization sequence) to importin α4 (KPNA3). ETV1, ETV4 and ETV5 mRNA levels in melanoma cells are elevated by siRNA (small interfering RNA) knockdown of capicúa, and decreased by inhibiting ERK and/or expressing a form of capicúa that cannot bind to 14-3-3 proteins. Capicúa knockdown also enhances cell migration. The findings of the present study give further mechanistic insights into why ETV1 is highly expressed in certain cancers, indicate that loss of capicúa can desensitize cells to the effects of ERK pathway inhibitors, and highlight interconnections among growth factor signalling, spinocerebellar ataxias and cancers.
cancer; capicúa; Ets translocation variant 1 (ETV1); 14-3-3 protein; spinocerebellar ataxia type 1 (SCA1); B2M, β2 microglobuluin; CRE, CIC-responsive element; DAPI, 4′,6-diamidino-2-phenylindole; DMEM, Dulbecco's modified Eagle's medium; DUX4, Double homeobox 4; ECL, enhanced chemiluminescence; EGF, epidermal growth factor; EMSA, electrophoretic mobility-shift assay; ERK, extracellular-signal-regulated kinase; ETV, Ets translocation variant; EWS, Ewing sarcoma protein; FBS, fetal bovine serum; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GFP, green fluorescent protein; GIST, gastrointestinal stromal tumour; HA, haemagglutinin; HEK, human embryonic kidney; HMG, high-mobility group; IGF1, insulin-like growth factor 1; KPNA3, importin α4/karyopherin α3; LC, liquid chromatography; MS/MS, tandem MS; NLS, nuclear localization sequence; p90RSK, p90 ribosomal S6 kinase; PEA3, polyoma enhancer activator 3; PDK1, phosphoinositide-dependent kinase 1; PI3K, phosphoinositide 3-kinase; PKB, protein kinase B; PKC, protein kinase C; RT, reverse transcription; SCA, spinocerebellar ataxia; siRNA, small interfering RNA
More than 200 phosphorylated 14-3-3-binding sites in the literature were analysed to define 14-3-3 specificities, identify relevant protein kinases, and give insights into how cellular 14-3-3/phosphoprotein networks work. Mode I RXX(pS/pT)XP motifs dominate, although the +2 proline residue occurs in less than half, and LX(R/K)SX(pS/pT)XP is prominent in plant 14-3-3-binding sites. Proline at +1 is rarely reported, and such motifs did not stand up to experimental reanalysis of human Ndel1. Instead, we discovered that 14-3-3 interacts with two residues that are phosphorylated by basophilic kinases and located in the DISC1 (disrupted-in-schizophrenia 1)-interacting region of Ndel1 that is implicated in cognitive disorders. These data conform with the general findings that there are different subtypes of 14-3-3-binding sites that overlap with the specificities of different basophilic AGC (protein kinase A/protein kinase G/protein kinase C family) and CaMK (Ca2+/calmodulin-dependent protein kinase) protein kinases, and a 14-3-3 dimer often engages with two tandem phosphorylated sites, which is a configuration with special signalling, mechanical and evolutionary properties. Thus 14-3-3 dimers can be digital logic gates that integrate more than one input to generate an action, and coincidence detectors when the two binding sites are phosphorylated by different protein kinases. Paired sites are generally located within disordered regions and/or straddle either side of functional domains, indicating how 14-3-3 dimers modulate the conformations and/or interactions of their targets. Finally, 14-3-3 proteins bind to members of several multi-protein families. Two 14-3-3-binding sites are conserved across the class IIa histone deacetylases, whereas other protein families display differential regulation by 14-3-3s. We speculate that 14-3-3 dimers may have contributed to the evolution of such families, tailoring regulatory inputs to different physiological demands.
14-3-3 protein; AGC protein kinase; Ca2+/calmodulin-dependent protein kinase; disrupted-in-schizophrenia 1 (DISC1); evolution; AANAT, serotonin acetyltransferase; AGC, protein kinase A/protein kinase G/protein kinase C family kinase; AMPK, AMP-activated protein kinase; BAD, Bcl-XL/Bcl-2-associated death promoter; CaMK, Ca2+/calmodulin-dependent protein kinase; CDK5, cyclin-dependent kinase 5; DIG, digoxigenin; DISC1, disrupted-in-schizophrenia 1; DSTT, Division of Signal Transduction Therapy; EST, expressed sequence tag; FOXO, Forkhead box O; GLUT4, glucose transporter 4; GST, glutathione transferase; HA, haemagglutinin; HAP1A, Huntingtin-associated protein 1A; HDAC, histone deacetylase; HEK, human embryonic kidney; KLC, kinesin light chain; MARK, microtubule affinity-regulating kinase; PI4K, phosphoinositide 4-kinase; PKB, protein kinase B; PKC, protein kinase C; PP2A, protein phosphatase 2A; RSK, ribosomal S6 kinase; YAP1, yes-associated protein 1
We devised a strategy of 14-3-3 affinity capture and release, isotope differential (d0/d4) dimethyl labeling of tryptic digests, and phosphopeptide characterization to identify novel targets of insulin/IGF1/phosphatidylinositol 3-kinase signaling. Notably four known insulin-regulated proteins (PFK-2, PRAS40, AS160, and MYO1C) had high d0/d4 values meaning that they were more highly represented among 14-3-3-binding proteins from insulin-stimulated than unstimulated cells. Among novel candidates, insulin receptor substrate 2, the proapoptotic CCDC6, E3 ubiquitin ligase ZNRF2, and signaling adapter SASH1 were confirmed to bind to 14-3-3s in response to IGF1/phosphatidylinositol 3-kinase signaling. Insulin receptor substrate 2, ZNRF2, and SASH1 were also regulated by phorbol ester via p90RSK, whereas CCDC6 and PRAS40 were not. In contrast, the actin-associated protein vasodilator-stimulated phosphoprotein and lipolysis-stimulated lipoprotein receptor, which had low d0/d4 scores, bound 14-3-3s irrespective of IGF1 and phorbol ester. Phosphorylated Ser19 of ZNRF2 (RTRAYpS19GS), phospho-Ser90 of SASH1 (RKRRVpS90QD), and phospho- Ser493 of lipolysis-stimulated lipoprotein receptor (RPRARpS493LD) provide one of the 14-3-3-binding sites on each of these proteins. Differential 14-3-3 capture provides a powerful approach to defining downstream regulatory mechanisms for specific signaling pathways.
Budding yeast (Saccharomyces cerevisiae) Slx4 is essential for cell viability in the absence of the Sgs1 helicase and for recovery from DNA damage. Here we report that cells lacking Slx4 have difficulties in completing DNA synthesis during recovery from replisome stalling induced by the DNA alkylating agent methyl methanesulfonate (MMS). Although DNA synthesis restarts during recovery, cells are left with unreplicated gaps in the genome despite an increase in translesion synthesis. In this light, epistasis experiments show that SLX4 interacts with genes involved in error-free bypass of DNA lesions. Slx4 associates physically, in a mutually exclusive manner, with two structure-specific endonucleases, Rad1 and Slx1, but neither of these enzymes is required for Slx4 to promote resistance to MMS. However, Rad1-dependent DNA repair by single-strand annealing (SSA) requires Slx4. Strikingly, phosphorylation of Slx4 by the Mec1 and Tel1 kinases appears to be essential for SSA but not for cell viability in the absence of Sgs1 or for cellular resistance to MMS. These results indicate that Slx4 has multiple functions in responding to DNA damage and that a subset of these are regulated by Mec1/Tel1-dependent phosphorylation.
Human (h)PTIP plays important but poorly understood roles in cellular responses to DNA damage. hPTIP interacts with 53BP1 tumour suppressor but only when 53BP1 is phosphorylated by ATM after DNA damage although the mechanism(s) and significance of the interaction of these two proteins are unclear. Here, we pinpoint a single ATM-phosphorylated residue in 53BP1—Ser25—that is required for binding of 53BP1 to hPTIP. Binding of phospho-Ser25 to hPTIP in vitro and in vivo requires two closely apposed pairs of BRCT domains at the C-terminus of hPTIP and neither pair alone can bind to phospho-Ser25, even though one of these BRCT pairs in isolation can bind to other ATM-phosphorylated epitopes. Mutations in 53BP1 and in hPTIP that prevent the interaction of the two proteins, render cells hypersensitive to DNA damage and weaken ATM signalling. The C-terminal BRCT domains of hPTIP are also required for stable retention of hPTIP at sites of DNA damage but this appears to be independent of binding to 53BP1. Thus, the BRCT domains of hPTIP play important roles in the cellular response to DNA damage.
Pim-1, 2 and 3 are a group of enzymes related to the calcium calmodulin family of protein kinases. Over-expression of Pim-1 and Pim-2 in mice promotes the development of lymphomas, and up-regulation of Pim expression has been observed in several human cancers.
Here we show that the pim kinases are constitutively active when expressed in HEK-293 cells and are able to phosphorylate the Bcl-2 family member Bad on three residues, Ser112, Ser136 and Ser155 in vitro and in cells. In vitro mapping showed that Pim-2 predominantly phosphorylated Ser112, while Pim-1 phosphorylated Ser112, but also Ser136 and Ser155 at a reduced rate compared to Ser112. Pim-3 was found to be the least specific for Ser112, and the most effective at phosphorylating Ser136 and Ser155. Pim-3 was also able to phosphorylate other sites in Bad in vitro, including Ser170, another potential in vivo site. Mutation of Ser136 to alanine prevented the phosphorylation of Ser112 and Ser155 by Pim kinases in HEK-293 cells, suggesting that this site must be phosphorylated first in order to make the other sites accessible. Pim phosphorylation of Bad was also found to promote the 14-3-3 binding of Bad and block its association with Bcl-XL.
All three Pim kinase family members predominantly phosphorylate Bad on Ser112 and in addition are capable of phosphorylating Bad on multiple sites associated with the inhibition of the pro-apoptotic function of Bad in HEK-293 cells. This would be consistent with the proposed function of Pim kinases in promoting cell proliferation and preventing cell death.
The soft rot bacteria Erwinia carotovora and Erwinia chrysanthemi are important pathogens of potato and other crops. However, the taxonomy of these pathogens, particularly at subspecies level, is unclear. An investigation using amplified fragment length polymorphism (AFLP) fingerprinting was undertaken to determine the taxonomic relationships within this group based on their genetic relatedness. Following cluster analysis on the similarity matrices derived from the AFLP gels, four clusters (clusters 1 to 4) resulted. Cluster 1 contained Erwinia carotovora subsp. carotovora (subclusters 1a and 1b) and Erwinia carotovora subsp. odorifera (subcluster 1c) strains, while cluster 2 contained Erwinia carotovora subsp. atroseptica (subcluster 2a) and Erwinia carotovora subsp. betavasculorum (subcluster 2b) strains. Clusters 3 and 4 contained Erwinia carotovora subsp. wasabiae and E. chrysanthemi strains, respectively. While E. carotovora subsp. carotovora and E. chrysanthemi showed a high level of molecular diversity (23 to 38% mean similarity), E. carotovora subsp. odorifera, E. carotovora subsp. betavasculorum, E. carotovora subsp. atroseptica, and E. carotovora subsp. wasabiae showed considerably less (56 to 76% mean similarity), which may reflect their limited geographical distributions and/or host ranges. The species- and subspecies-specific banding profiles generated from the AFLPs allowed rapid identification of unknown isolates and the potential for future development of diagnostics. AFLP fingerprinting was also found to be more differentiating than other techniques for typing the soft rot erwinias and was applicable to all strain types, including different serogroups.