Efficacy of Systemically Administered Retargeted Oncolytic Herpes Simplex Viruses-Clearance and Biodistribution in Naïve and HSV-Preimmune Mice.

We investigated the anticancer efficacy, blood clearance, and tissue biodistribution of systemically administered retargeted oncolytic herpes simplex viruses (ReHVs) in HSV-naïve and HSV-preimmunized (HSV-IMM) mice. Efficacy was tested against lung tumors formed upon intravenous administration of cancer cells, a model of metastatic disease, and against subcutaneous distant tumors. In naïve mice, HER2- and hPSMA-retargeted viruses, both armed with mIL-12, were highly effective, even when administered to mice with well-developed tumors. Efficacy was higher for combination regimens with immune checkpoint inhibitors. A significant amount of infectious virus persisted in the blood for at least 1 h. Viral genomes, or fragments thereof, persisted in the blood and tissues for days. Remarkably, the only sites of viral replication were the lungs of tumor-positive mice and the subcutaneous tumors. No replication was detected in other tissues, strengthening the evidence of the high cancer specificity of ReHVs, a property that renders ReHVs suitable for systemic administration. In HSV-IMM mice, ReHVs administered at late times failed to exert anticancer efficacy, and the circulating virus was rapidly inactivated. Serum stability and in vivo whole blood stability assays highlighted neutralizing antibodies as the main factor in virus inactivation. Efforts to deplete mice of the neutralizing antibodies are ongoing.

Towards a Precision Medicine Approach and In Situ Vaccination against Prostate Cancer by PSMA-Retargeted oHSV.

Prostate specific membrane antigen (PSMA) is a specific high frequency cell surface marker of prostate cancers. Theranostic approaches targeting PSMA show no major adverse effects and rule out off-tumor toxicity. A PSMA-retargeted oHSV (R-405) was generated which both infected and was cytotoxic exclusively for PSMA-positive cells, including human prostate cancer LNCaP and 22Rv1 cells, and spared PSMA-negative cells. R-405 in vivo efficacy against LLC1-PSMA and Renca-PSMA tumors consisted of inhibiting primary tumor growth, establishing long-term T immune response, immune heating of the microenvironment, de-repression of the anti-tumor immune phenotype, and sensitization to checkpoint blockade. The in situ vaccination protected from distant challenge tumors, both PSMA-positive and PSMA-negative, implying that it was addressed also to LLC1 tumor antigens. PSMA-retargeted oHSVs are a precision medicine tool worth being additionally investigated in the immunotherapeutic and in situ vaccination landscape against prostate cancers.

Genotype of Immunologically Hot or Cold Tumors Determines the Antitumor Immune Response and Efficacy by Fully Virulent Retargeted oHSV.

We report on the efficacy of the non-attenuated HER2-retargeted oHSV named R-337 against the immunologically hot CT26-HER2 tumor, and an insight into the basis of the immune protection. Preliminarily, we conducted an RNA immune profiling and immune cell content characterization of CT26-HER2 tumor in comparison to the immunologically cold LLC1-HER2 tumor. CT26-HER2 tumor was implanted into HER2-transgenic BALB/c mice. Hallmarks of R-337 effects were the protection from primary tumor, long-term adaptive vaccination directed to both HER2 and CT26-wt cell neoantigens. The latter effect differentiated R-337 from OncoVEXGM-CSF. As to the basis of the immune protection, R-337 orchestrated several changes to the tumor immune profile, which cumulatively reversed the immunosuppression typical of this tumor (graphical abstract). Thus, Ido1 (inhibitor of T cell anticancer immunity) levels and T regulatory cell infiltration were decreased; Cd40 and Cd27 co-immunostimulatory markers were increased; the IFNγ cascade was activated. Of note was the dampening of IFN-I response, which we attribute to the fact that R-337 is fully equipped with genes that contrast the host innate response. The IFN-I shut-down likely favored viral replication and the expression of the mIL-12 payload, which, in turn, boosted the antitumor response. The results call for a characterization of tumor immune markers to employ oncolytic herpesviruses more precisely.

Therapeutic potential of targeting mTOR in T-cell acute lymphoblastic leukemia (review).

T-cell acute lymphoblastic leukemia (T-ALL) is a heterogeneous neoplastic disorder of immature hematopoietic precursors committed to the T-cell lineage. T-ALL comprises about 15% of pediatric and 25% of adult ALL cases. Even if the prognosis of T-ALL has improved especially in the childhood due to the use of new intensified treatment protocols, the outcome of relapsed patients who are resistant to conventional chemotherapeutic drugs or who relapse is still poor. For this reason, there is a need for novel and less toxic targeted therapies against signaling pathways aberrantly activated in T-ALL, such as the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR). Small molecules designed to target key components of this signaling axis have proven their efficacy both in vitro and in vivo in pre-clinical settings of T-ALL. In particular, different classes of mTOR inhibitors have been disclosed by pharmaceutical companies, and they are currently being tested in clinical trials for treating T-ALL patients. One of the most promising approaches for the treatment of T-ALL seems to be the combination of mTOR inhibitors with traditional chemotherapeutic agents. This could lead to a lower drug dosage that may circumvent the systemic side effects of chemotherapeutics. In this review, we focus on the different classes of mTOR inhibitors that will possibly have an impact on the therapeutic arsenal we have at our disposal against T-ALL.

Targeting signaling pathways in T-cell acute lymphoblastic leukemia initiating cells.

Leukemia initiating cells (LICs) represent a reservoir that is believed to drive relapse and resistance to chemotherapy in blood malignant disorders. T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive neoplastic disorder of immature hematopoietic precursors committed to the T-cell lineage. T-ALL comprises about 15% of pediatric and 25% of adult ALL cases and is prone to early relapse. Although the prognosis of T-ALL has improved especially in children due to the use of new intensified treatment protocols, the outcome of relapsed T-ALL cases is still poor. Putative LICs have been identified also in T-ALL. LICs are mostly quiescent and for this reason highly resistant to chemotherapy. Therefore, they evade treatment and give rise to disease relapse. At present great interest surrounds the development of targeted therapies against signaling networks aberrantly activated in LICs and important for their survival and drug-resistance. Both the Notch1 pathway and the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) network are involved in T-ALL LIC survival and drug-resistance and could be targeted by small molecules. Thus, Notch1 and PI3K/Akt/mTOR inhibitors are currently being developed for clinical use either as single agents or in combination with conventional chemotherapy for T-ALL patient treatment. In this review, we summarize the existing knowledge of the relevance of Notch1 and PI3K/Akt/mTOR signaling in T-ALL LICs and we examine the rationale for targeting these key signal transduction networks by means of selective pharmacological inhibitors.

Therapeutic targeting of Polo-like kinase-1 and Aurora kinases in T-cell acute lymphoblastic leukemia.

Polo-like kinases (PLKs) and Aurora kinases (AKs) act as key cell cycle regulators in healthy human cells. In cancer, these protein kinases are often overexpressed and dysregulated, thus contributing to uncontrolled cell proliferation and growth. T-cell acute lymphoblastic leukemia (T-ALL) is a heterogeneous malignancy arising in the thymus from T-cell progenitors. Primary chemoresistant and relapsed T-ALL patients have yet a poor outcome, therefore novel therapies, targeting signaling pathways important for leukemic cell proliferation, are required. Here, we demonstrate the potential therapeutic effects of BI6727, MK-5108, and GSK1070916, three selective inhibitors of PLK1, AK-A, and AK-B/C, respectively, in a panel of T-ALL cell lines and primary cells from T-ALL patients. The drugs were both cytostatic and cytotoxic to T-ALL cells by inducing G2/M-phase arrest and apoptosis. The drugs retained part of their pro-apoptotic activity in the presence of MS-5 bone marrow stromal cells. Moreover, we document for the first time that BI6727 perturbed both the PI3K/Akt/mTORC2 and the MEK/ERK/mTORC1 signaling pathways, and that a combination of BI6727 with specific inhibitors of the aforementioned pathways (MK-2206, CCI-779) displayed significantly synergistic cytotoxic effects. Taken together, our findings indicate that PLK1 and AK inhibitors display the potential for being employed in innovative therapeutic strategies for improving T-ALL patient outcome.

MYCN is a novel oncogenic target in pediatric T-cell acute lymphoblastic leukemia.

MYCN is an oncogene frequently overexpressed in pediatric solid tumors whereas few evidences suggest his involvement in the pathogenesis of haematologic malignancies. Here we show that MYCN is overexpressed in a relevant proportion (40 to 50%) of adult and pediatric T-cell acute lymphoblastic leukemias (T-ALL). Focusing on pediatric T-ALL, MYCN-expressing samples were found almost exclusively in the TAL1-positive subgroup. Moreover, TAL1 knockdown in T-ALL cell lines resulted in a reduction of MYCN expression, and TAL1 directly binds to MYCN promoter region, suggesting that TAL1 pathway activation could sustain the up-regulation of MYCN. The role of MYCN in T-ALL was investigated by peptide nucleic acid (PNA-MYCN)-mediated transcriptional silencing of MYCN and by siRNAs. MYCN knockdown in T-ALL cell lines resulted in a reduction of cell viability, up to 50%, while no effect was elicited with a mismatch PNA. The inhibitory effect of PNA-MYCN on cell viability was due to a significant increase in apoptosis. PNA-MYCN treatment in pediatric T-ALL samples reduced cell viability of leukemic cells from patients with high MYCN expression, while no effect was obtained in MYCN-negative blast cells. These results showed that MYCN is frequently overexpressed in pediatric T-ALL and suggested his role as a candidate for molecularly-directed therapies.

Targeting phosphatidylinositol 3-kinase signaling in acute myelogenous leukemia.

INTRODUCTION: Despite continuous advances in our knowledge of the biology of acute myelogenous leukemia (AML), the prognosis of AML patients treated with standard chemotherapy is still poor, especially in the elderly. Therefore, there is a need for novel targeted and less toxic therapies, particularly for patients who develop resistance to traditional chemotherapeutic drugs. Constitutively active phosphatidylinositol 3-kinase (PI3K) signaling characterizes many types of tumors, including AML, where it negatively influences response to therapeutic treatments. AREAS COVERED: The literature data showed that small inhibitor molecules targeting PI3K signaling induced cell cycle arrest, apoptosis and decreased drug-resistance in AML cells. PI3K inhibitors were also capable of targeting leukemic initiating cells (LICs), the most relevant target for leukemia eradication, whereas they tended to spare healthy hematopoietic stem cells. EXPERT OPINION: Data emerging from pre-clinical settings suggest that the PI3K pathway is critically involved in regulating proliferation, survival and drug-resistance of AML cells. Therefore, we propose that novel drugs targeting this signaling pathway may offer a novel and less toxic treatment option for AML patients, most likely in combination with a lower dosage of traditional chemotherapeutic agents or other innovative therapeutic agents.

A combination of temsirolimus, an allosteric mTOR inhibitor, with clofarabine as a new therapeutic option for patients with acute myeloid leukemia.

Signaling through the phosphatidylinositol 3-kinase (PI3K) pathway and its downstream effectors, Akt and mechanistic target of rapamycin (mTOR), is aberrantly activated in acute myeloid leukemia (AML) patients, where it contributes to leukemic cell proliferation, survival, and drug-resistance. Thus, inhibiting mTOR signaling in AML blasts could enhance their sensitivity to cytotoxic agents. Preclinical data also suggest that allosteric mTOR inhibition with rapamycin impaired leukemia initiating cells (LICs) function. In this study, we assessed the therapeutic potential of a combination consisting of temsirolimus [an allosteric mTOR complex 1 (mTORC1) inhibitor] with clofarabine, a nucleoside analogue with potent inhibitory effects on both ribonucleotide reductase and DNA polymerase. The drug combination (CLO-TOR) displayed synergistic cytotoxic effects against a panel of AML cell lines and primary cells from AML patients. Treatment with CLO-TOR induced a G0/G1-phase cell cycle arrest, apoptosis, and autophagy. CLO-TOR was pro-apoptotic in an AML patient blast subset (CD34⁺/CD38⁻/CD123⁺), which is enriched in putative leukemia initiating cells (LICs). In summary, the CLO-TOR combination could represent a novel valuable treatment for AML patients, also in light of its efficacy against LICs.

The emerging multiple roles of nuclear Akt.

Akt is a central player in the signal transduction pathways activated in response to many growth factors, hormones, cytokines, and nutrients and is thought to control a myriad of cellular functions including proliferation and survival, autophagy, metabolism, angiogenesis, motility, and exocytosis. Moreover, dysregulated Akt activity is being implicated in the pathogenesis of a growing number of disorders, including cancer. Evidence accumulated over the past 15 years has highlighted the presence of active Akt in the nucleus, where it acts as a fundamental component of key signaling pathways. For example, nuclear Akt counteracts apoptosis through a block of caspase-activated DNase: deoxyribonuclease and inhibition of chromatin condensation, and is also involved in cell cycle progression control, cell differentiation, mRNA: messenger RNA export, DNA repair, and tumorigenesis. In this review, we shall summarize the most relevant findings about nuclear Akt and its functions.

Harnessing the PI3K/Akt/mTOR pathway in T-cell acute lymphoblastic leukemia: eliminating activity by targeting at different levels.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignant hematological disorder arising in the thymus from T-cell progenitors. T-ALL mainly affects children and young adults, and remains fatal in 20% of adolescents and 50% of adults, despite progress in polychemotherapy protocols. Therefore, innovative targeted therapies are desperately needed for patients with a dismal prognosis. Aberrant activation of PI3K/Akt/mTOR signaling is a common event in T-ALL patients and portends a poor prognosis. Preclinical studies have highlighted that modulators of PI3K/Akt/mTOR signaling could have a therapeutic relevance in T-ALL. However, the best strategy for inhibiting this highly complex signal transduction pathway is still unclear, as the pharmaceutical companies have disclosed an impressive array of small molecules targeting this signaling network at different levels. Here, we demonstrate that a dual PI3K/PDK1 inhibitor, NVP-BAG956, displayed the most powerful cytotoxic affects against T-ALL cell lines and primary patients samples, when compared with a pan class I PI3K inhibitor (GDC-0941), an allosteric Akt inhibitor (MK-2206), an mTORC1 allosteric inhibitor (RAD-001), or an ATP-competitive mTORC1/mTORC2 inhibitor (KU63794). Moreover, we also document that combinations of some of the aforementioned drugs strongly synergized against T-ALL cells at concentrations well below their respective IC50. This observation indicates that vertical inhibition at different levels of the PI3K/Akt/mTOR network could be considered as a future innovative strategy for treating T-ALL patients.

Activity of the selective IκB kinase inhibitor BMS-345541 against T-cell acute lymphoblastic leukemia: involvement of FOXO3a.

Several lines of evidence suggest that the IκB kinase (IKK)/nuclear factor-κB (NFκB) axis is required for viability of leukemic cells and is a predictor of relapse in T-cell acute lymphoblastic leukemia (T-ALL). Moreover, many anticancer agents induce NFκB nuclear translocation and activation of its target genes, which counteract cellular resistance to chemotherapeutic drugs. Therefore, the design and the study of IKK-specific drugs is crucial to inhibit tumor cell proliferation and to prevent cancer drug-resistance. Here, we report the anti-proliferative effects induced by BMS-345541 (a highly selective IKK inhibitor) in three Notch1-mutated T-ALL cell lines and in T-ALL primary cells from pediatric patients. BMS-345541 induced apoptosis and an accumulation of cells in the G 2/M phase of the cell cycle via inhibition of IKK/NFκB signaling. We also report that T-ALL cells treated with BMS-345541 displayed nuclear translocation of FOXO3a and restoration of its functions, including control of p21(Cip1) expression levels. We demonstrated that FOXO3a subcellular re-distribution is independent of AKT and ERK 1/2 signaling, speculating that in T-ALL the loss of FOXO3a tumor suppressor function could be due to deregulation of IKK, as has been previously demonstrated in other cancer types. It is well known that, differently from p53, FOXO3a mutations have not yet been found in human tumors, which makes therapeutics activating FOXO3a more appealing than others. For these features, BMS-345541 could be used alone or in combination with traditional therapies in the treatment of T-ALL.

Targeting the liver kinase B1/AMP-activated protein kinase pathway as a therapeutic strategy for hematological malignancies.

INTRODUCTION: Despite considerable advances, several hematological malignancies remain incurable with standard treatments. Therefore, there is a need for novel targeted and less toxic therapies, particularly for patients who develop resistance to traditional chemotherapeutic drugs. The liver kinase B1 (LKB1)/AMP-activated protein kinase (AMPK) signaling pathway has recently emerged as a tumor suppressor axis. A critical point is that the LKB1/AMPK network remains functional in a wide range of cancers and could be stimulated by drugs, such as N,N-dimethylimidodicarbonimidic diamide (metformin) or 5-aminoimidazole-4-carboxamide 1-ß-D-ribofuranoside (AICAR). AREAS COVERED: The literature data show that drugs activating LKB1/AMPK signaling induced cell cycle arrest, caspase-dependent apoptosis or autophagy in hematopoietic tumors. Moreover, metformin effectively inhibited mammalian target of rapamycin complex 1 (mTORC1)-controlled oncogenetic protein translation, which does not occur with allosteric mTORC1 inhibitors, such as rapamycin and its derivatives. Metformin was also capable of targeting leukemic stem cells, the most relevant target for leukemia eradication. EXPERT OPINION: Data emerging from preclinical settings suggest that the LKB1/AMPK pathway is critically involved in regulating proliferation and survival of malignant hematopoietic cells. Thus, it is proposed that drugs activating the LKB1/AMPK axis may offer a novel and less toxic treatment option for some types of hematological malignancies.

Stabilization of the Escherichia coli DNA polymerase III ε subunit by the θ subunit favors in vivo assembly of the Pol III catalytic core.

Escherichia coli DNA polymerase III holoenzyme (HE) contains a core polymerase consisting of three subunits: α (polymerase), ε (3'-5' exonuclease), and θ. Genetic experiments suggested that θ subunit stabilizes the intrinsically labile ε subunit and, furthermore, that θ might affect the cellular amounts of Pol III core and HE. Here, we provide biochemical evidence supporting this model by analyzing the amounts of the relevant proteins. First, we show that a ΔholE strain (lacking θ subunit) displays reduced amounts of free ε. We also demonstrate the existence of a dimer of ε, which may be involved in the stabilization of the protein. Second, θ, when overexpressed, dissociates the ε dimer and significantly increases the amount of Pol III core. The stability of ε also depends on cellular chaperones, including DnaK. Here, we report that: (i) temperature shift-up of ΔdnaK strains leads to rapid depletion of ε, and (ii) overproduction of θ overcomes both the depletion of ε and the temperature sensitivity of the strain. Overall, our data suggest that ε is a critical factor in the assembly of Pol III core, and that this is role is strongly influenced by the θ subunit through its prevention of ε degradation.

Two hits are better than one: targeting both phosphatidylinositol 3-kinase and mammalian target of rapamycin as a therapeutic strategy for acute leukemia treatment.

Phosphatidylinositol 3-kinase (PI3K) and mammalian target of rapamycin (mTOR) are two key components of the PI3K/Akt/mTOR signaling pathway. This signal transduction cascade regulates a wide range of physiological cell processes, that include differentiation, proliferation, apoptosis, autophagy, metabolism, motility, and exocytosis. However, constitutively active PI3K/Akt/mTOR signaling characterizes many types of tumors where it negatively influences response to therapeutic treatments. Hence, targeting PI3K/Akt/mTOR signaling with small molecule inhibitors may improve cancer patient outcome. The PI3K/Akt/mTOR signaling cascade is overactive in acute leukemias, where it correlates with enhanced drug-resistance and poor prognosis. The catalytic sites of PI3K and mTOR share a high degree of sequence homology. This feature has allowed the synthesis of ATP-competitive compounds targeting the catalytic site of both kinases. In preclinical models, dual PI3K/mTOR inhibitors displayed a much stronger cytotoxicity against acute leukemia cells than either PI3K inhibitors or allosteric mTOR inhibitors, such as rapamycin. At variance with rapamycin, dual PI3K/mTOR inhibitors targeted both mTOR complex 1 and mTOR complex 2, and inhibited the rapamycin-resistant phosphorylation of eukaryotic initiation factor 4E-binding protein 1, resulting in a marked inhibition of oncogenic protein translation. Therefore, they strongly reduced cell proliferation and induced an important apoptotic response. Here, we reviewed the evidence documenting that dual PI3K/mTOR inhibitors may represent a promising option for future targeted therapies of acute leukemia patients.

Spermidine application to young developing peach fruits leads to a slowing down of ripening by impairing ripening-related ethylene and auxin metabolism and signaling.

Peach (Prunus persica var. laevis Gray) was chosen to unravel the molecular basis underlying the ability of spermidine (Sd) to influence fruit development and ripening. Field applications of 1 mM Sd on peach fruit at an early developmental stage, 41 days after full bloom (dAFB), i.e. at late stage S1, led to a slowing down of fruit ripening. At commercial harvest (125 dAFB, S4II) Sd-treated fruits showed a reduced ethylene production and flesh softening. The endogenous concentration of free and insoluble conjugated polyamines (PAs) increased (0.3-2.6-fold) 1 day after treatment (short-term response) butsoon it declined to control levels; starting from S3/S4, when soluble conjugated forms increased (up to five-fold relative to controls at ripening), PA levels became more abundant in treated fruits, (long-term response). Real-time reverse transcription-polymerase chain reaction analyses revealed that peaks in transcript levels of fruit developmental marker genes were shifted ahead in accord with a developmental slowing down. At ripening (S4I-S4II) the upregulation of the ethylene biosynthetic genes ACO1 and ACS1 was dramatically counteracted by Sd and this led to a strong downregulation of genes responsible for fruit softening, such as PG and PMEI. Auxin-related gene expression was also altered both in the short term (TRPB) and in the long term (GH3, TIR1 and PIN1), indicating that auxin plays different roles during development and ripening processes. Messenger RNA amounts of other hormone-related ripening-regulated genes, such as NCED and GA2-OX, were strongly downregulated at maturity. Results suggest that Sd interferes with fruit development/ripening by interacting with multiple hormonal pathways.

Nuclear phosphoinositides and their roles in cell biology and disease.

Since the late 1980s, a growing body of evidence has documented that phosphoinositides and their metabolizing enzymes, which regulate a large variety of cellular functions both in the cytoplasm and at the plasma membrane, are present also within the nucleus, where they are involved in processes such as cell proliferation, differentiation, and survival. Remarkably, nuclear phosphoinositide metabolism operates independently from that present elsewhere in the cell. Although nuclear phosphoinositides generate second messengers such as diacylglycerol and inositol 1,4,5 trisphosphate, it is becoming increasingly clear that they may act by themselves to influence chromatin structure, gene expression, DNA repair, and mRNA export. The understanding of the biological roles played by phosphoinositides is supported by the recent acquisitions demonstrating the presence in the nuclear compartment of several proteins harboring phosphoinositide-binding domains. Some of these proteins have functional roles in RNA splicing/processing and chromatin assembly. Moreover, recent evidence shows that nuclear phospholipase Cß1 (a key phosphoinositide metabolizing enzyme) could somehow be involved in the myelodysplastic syndrome, i.e. a hematopoietic disorder that frequently evolves into an acute leukemia. This review aims to highlight the most significant and updated findings about phosphoinositide metabolism in the nucleus under both physiological and pathological conditions.

Shine-Dalgarno sequence enhances the efficiency of lacZ repression by artificial anti-lac antisense RNAs in Escherichia coli.

Silencing of the lacZ gene in Escherichia coli was attempted by means of the expression of antisense RNAs (asRNAs) in vivo. A short fragment of lacZ was cloned into the pBAD expression vector, in reverse orientation, using the EcoRI and PstI restriction sites. This construct (pBAD-Zcal1) was used to transform E. coli cells, and the antisense transcription was induced simply by adding arabinose to the culture medium. We demonstrated that the Zcal1 asRNA effectively silenced lacZ using ß-galactosidase activity determinations, SDS-PAGE, and Western blotting. Because the concentration of the lac mRNA was always high in cells that expressed Zcal1, we hypothesize that this antisense acts by inhibiting messenger translation. Similar analyses, performed with a series of site-specific Zcal1 mutants, showed that the Shine-Dalgarno sequence, which is conferred by the pBAD vector, is an essential requisite for silencing competence. Indeed, the presence of the intact Shine-Dalgarno sequence positively affects asRNA stability and, hence, silencing effectiveness. Our observations will contribute to the understanding of the main determinants of silencing as exerted by asRNAs as well as provide useful support for the design of robust and efficient prokaryotic gene silencers.

Proteolysis of the proofreading subunit controls the assembly of Escherichia coli DNA polymerase III catalytic core.

The C-terminal region of the proofreading subunit (epsilon) of Escherichia coli DNA polymerase III is shown here to be labile and to contain the residues (identified between F187 and R213) responsible for association with the polymerase subunit (alpha). We also identify two alpha-helices of the polymerase subunit (comprising the residues E311-M335 and G339-D353, respectively) as the determinants of binding to epsilon. The C-terminal region of epsilon is degraded by the ClpP protease assisted by the GroL molecular chaperone, while other factors control the overall concentration in vivo of epsilon. Among these factors, the chaperone DnaK is of primary importance for preserving the integrity of epsilon. Remarkably, inactivation of DnaK confers to Escherichia coli inviable phenotype at 42 degrees C, and viability can be restored over-expressing epsilon. Altogether, our observations indicate that the association between epsilon and alpha subunits of DNA polymerase III depends on small portions of both proteins, the association of which is controlled by proteolysis of epsilon. Accordingly, the factors catalysing (ClpP, GroL) or preventing (DnaK) this proteolysis exert a crucial checkpoint of the assembly of Escherichia coli DNA polymerase III core.