DNA polymerase lambda directly binds to proliferating cell nuclear antigen through its confined C-terminal region

Saccharomyces cerevisiae DNA polymerase IV overcomes Rad51 inhibition of DNA polymerase δ in Rad52-mediated direct-repeat recombination

Saccharomyces cerevisiae DNA polymerase IV (Pol4) like its homolog, human DNA polymerase lambda (Polλ), is involved in Non-Homologous End-Joining and Microhomology-Mediated Repair. Using genetic analysis, we identified an additional role of Pol4 also in homology-directed DNA repair, specifically in Rad52-dependent/Rad51-independent direct-repeat recombination. Our results reveal that the requirement for Pol4 in repeat recombination was suppressed by the absence of Rad51, suggesting that Pol4 counteracts the Rad51 inhibition of Rad52-mediated repeat recombination events. Using purified proteins and model substrates, we reconstituted in vitro reactions emulating DNA synthesis during direct-repeat recombination and show that Rad51 directly inhibits Polδ DNA synthesis. Interestingly, although Pol4 was not capable of performing extensive DNA synthesis by itself, it aided Polδ in overcoming the DNA synthesis inhibition by Rad51. In addition, Pol4 dependency and stimulation of Polδ DNA synthesis in the presence of Rad51 occurred in reactions containing Rad52 and RPA where DNA strand-annealing was necessary. Mechanistically, yeast Pol4 displaces Rad51 from ssDNA independent of DNA synthesis. Together our in vitro and in vivo data suggest that Rad51 suppresses Rad52-dependent/Rad51-independent direct-repeat recombination by binding to the primer-template and that Rad51 removal by Pol4 is critical for strand-annealing dependent DNA synthesis.

Revisiting the Function of p21CDKN1A in DNA Repair: The Influence of Protein Interactions and Stability

The p21CDKN1A protein is an important player in the maintenance of genome stability through its function as a cyclin-dependent kinase inhibitor, leading to cell-cycle arrest after genotoxic damage. In the DNA damage response, p21 interacts with specific proteins to integrate cell-cycle arrest with processes such as transcription, apoptosis, DNA repair, and cell motility. By associating with Proliferating Cell Nuclear Antigen (PCNA), the master of DNA replication, p21 is able to inhibit DNA synthesis. However, to avoid conflicts with this process, p21 protein levels are finely regulated by pathways of proteasomal degradation during the S phase, and in all the phases of the cell cycle, after DNA damage. Several lines of evidence have indicated that p21 is required for the efficient repair of different types of genotoxic lesions and, more recently, that p21 regulates DNA replication fork speed. Therefore, whether p21 is an inhibitor, or rather a regulator, of DNA replication and repair needs to be re-evaluated in light of these findings. In this review, we will discuss the lines of evidence describing how p21 is involved in DNA repair and will focus on the influence of protein interactions and p21 stability on the efficiency of DNA repair mechanisms.

'PIPs' in DNA polymerase: PCNA interaction affairs

Interaction of PCNA with DNA polymerase is vital to efficient and processive DNA synthesis. PCNA being a homotrimeric ring possesses three hydrophobic pockets mostly involved in an interaction with its binding partners. PCNA interacting proteins contain a short sequence of eight amino acids, popularly coined as PIP motif, which snuggly fits into the hydrophobic pocket of PCNA to stabilize the interaction. In the last two decades, several PIP motifs have been mapped or predicted in eukaryotic DNA polymerases. In this review, we summarize our understandings of DNA polymerase-PCNA interaction, the function of such interaction during DNA synthesis, and emphasize the lacunae that persist. Because of the presence of multiple ligands in the replisome complex and due to many interaction sites in DNA polymerases, we also propose two modes of DNA polymerase positioning on PCNA required for DNA synthesis to rationalize the tool-belt model of DNA replication.

Noncatalytic, N-terminal Domains of DNA Polymerase Lambda Affect Its Cellular Localization and DNA Damage Response

Specialized DNA polymerases, such as DNA polymerase lambda (Polλ), are important players in DNA damage tolerance and repair pathways. Knowing how DNA polymerases are regulated and recruited to sites of DNA damage is imperative to understanding these pathways. Recent work has suggested that Polλ plays a role in several distinct DNA damage tolerance and repair pathways. In this paper, we report previously unknown roles of the N-terminal domains of human Polλ for modulating its involvement in DNA damage tolerance and repair. By using Western blot analysis, fluorescence microscopy, and cell survival assays, we found that the BRCA1 C-terminal (BRCT) and proline/serine-rich (PSR) domains of Polλ affect its cellular localization and DNA damage responses. The nuclear localization signal (NLS) of Polλ was necessary to overcome the impediment of its nuclear localization caused by its BRCT and PSR domains. Induction of DNA damage resulted in recruitment of Polλ to chromatin, which was controlled by its BRCT and PSR domains. In addition, the presence of both domains was required for Polλ-mediated tolerance of oxidative DNA damage but not DNA methylation damage. These findings suggest that the N-terminal domains of Polλ are important for regulating its responses to DNA damage.

Anticancer Activity of Stilbene-Based Derivatives

Stilbene is an abundant structural scaffold in nature, and stilbene-based compounds have been widely reported for their biological activity. Notably, (E)-resveratrol and its natural stilbene-containing derivatives have been extensively investigated as cardioprotective, potent antioxidant, anti-inflammatory, and anticancer agents. Starting from its potent chemotherapeutic activity against a wide variety of cancers, the stilbene scaffold has been subject to synthetic manipulations with the aim of obtaining new analogues with improved anticancer activity and better bioavailability. Within the last decade, the majority of new synthetic stilbene derivatives have demonstrated significant anticancer activity against a large number of cancer cell lines, depending on the type and position of substituents on the stilbene skeleton. This review focuses on the structure-activity relationship of the key compounds containing a stilbene scaffold and describes how the structural modifications affect their anticancer activity.

Early nucleosome deposition on, and replication of, HSV DNA requires cell factor PCNA

Herpes simplex virus (HSV) is a double-stranded DNA virus that can cause lytic infections in epithelial cells of the skin and latent infections in neuronal cells of the peripheral nervous system. After virion attachment to the cell membrane, the capsid enters the cytoplasm and is transported to the nucleus. Following docking at the nuclear pore, the HSV DNA, and contents of the virion, are injected into the nucleus. The viral DNA that enters the nucleus is devoid of histones, but begins to be covered with them soon after entry. The covering of histones, in the form of nucleosomes, reaches a maximum during the early stages of infection and drops off during late infection (after DNA replication). However, during latency, the genome is saturated with nucleosomes. In this study, we examine the role of proliferating cell nuclear antigen (PCNA), a cellular DNA polymerase accessory protein (processivity factor), and cell DNA polymerases in histone deposition during the early stages of HSV infection. Using SiRNA knockdown, and a cytosine arabinoside (araC) chemical inhibitor, we conclude that PCNA is important for viral replication and histone deposition. However, cell DNA polymerases that bind PCNA do not appear to be required for these processes and PCNA does not appear to bind to the viral DNA polymerase (which has its own viral processivity factor).

Clustered DNA lesions containing 5-formyluracil and AP site: repair via the BER system

Lesions in the DNA arise under ionizing irradiation conditions or various chemical oxidants as a single damage or as part of a multiply damaged site within 1–2 helical turns (clustered lesion). Here, we explored the repair opportunity of the apurinic/apyrimidinic site (AP site) composed of the clustered lesion with 5-formyluracil (5-foU) by the base excision repair (BER) proteins. We found, that if the AP site is shifted relative to the 5-foU of the opposite strand, it could be repaired primarily via the short-patch BER pathway. In this case, the cleavage efficiency of the AP site-containing DNA strand catalyzed by human apurinic/apyrimidinic endonuclease 1 (hAPE1) decreased under AP site excursion to the 3'-side relative to the lesion in the other DNA strand. DNA synthesis catalyzed by DNA polymerase lambda was more accurate in comparison to the one catalyzed by DNA polymerase beta. If the AP site was located exactly opposite 5-foU it was expected to switch the repair to the long-patch BER pathway. In this situation, human processivity factor hPCNA stimulates the process.

DNA polymerases beta and lambda mediate overlapping and independent roles in base excision repair in mouse embryonic fibroblasts

Base excision repair (BER) is a DNA repair pathway designed to correct small base lesions in genomic DNA. While DNA polymerase beta (pol β) is known to be the main polymerase in the BER pathway, various studies have implicated other DNA polymerases in back-up roles. One such polymerase, DNA polymerase lambda (pol λ), was shown to be important in BER of oxidative DNA damage. To further explore roles of the X-family DNA polymerases λ and β in BER, we prepared a mouse embryonic fibroblast cell line with deletions in the genes for both pol β and pol λ. Neutral red viability assays demonstrated that pol λ and pol β double null cells were hypersensitive to alkylating and oxidizing DNA damaging agents. In vitro BER assays revealed a modest contribution of pol λ to single-nucleotide BER of base lesions. Additionally, using co-immunoprecipitation experiments with purified enzymes and whole cell extracts, we found that both pol λ and pol β interact with the upstream DNA glycosylases for repair of alkylated and oxidized DNA bases. Such interactions could be important in coordinating roles of these polymerases during BER.

DNA polymerases beta and lambda bypass thymine glycol in gapped DNA structures

Here we investigated the ability of the human X-family DNA polymerases beta and lambda to bypass thymine glycol (Tg) in gapped DNA substrates with the damage located in a defined position of the template strand. Maximum velocities and the Michaelis constant values were determined to study DNA synthesis in the presence of either Mg(2+) or Mn(2+). Additionally, the influence of hRPA (human replication protein A) and hPCNA (human proliferating cell nuclear antigen) on TLS (translesion synthesis) activity of DNA polymerases beta and lambda was examined. The results show that (i) DNA polymerase lambda is able to catalyze DNA synthesis across Tg, (ii) the ability of DNA polymerase lambda to elongate from a base paired to a Tg lesion is influenced by the size of the DNA gap, (iii) hPCNA increases the fidelity of Tg bypass and does not influence normal DNA synthesis catalyzed by DNA polymerase lambda, (iv) DNA polymerase beta catalyzes the incorporation of all four dNTPs opposite Tg, and (v) hPCNA as well as hRPA has no specific effect on TLS in comparison with the normal DNA synthesis catalyzed by DNA polymerase beta. These results considerably extend our knowledge concerning the ability of specialized DNA polymerases to cope with a very common DNA lesion such as Tg.

Silibinin exerts sustained growth suppressive effect against human colon carcinoma SW480 xenograft by targeting multiple signaling molecules

PURPOSE

Earlier, we reported the strong preventive efficacy of silibinin against colorectal cancer (CRC), but its usefulness against established CRC or effect of its withdrawal on CRC growth remained unknown. Present study focused on these important issues by employing two different treatment protocols in advanced human CRC SW480 xenograft in nude mice.

METHODS

In the first treatment protocol, silibinin was fed for 28 days (200 mg/kg body weight, 5 days/week) to mice with growing SW480 xenograft; thereafter, tumor growth was monitored for additional 3 weeks without silibinin treatment. In the second protocol, silibinin treatment was started after 25 days of SW480 cells injection (established tumors), and tumor growth was studied 4 days, 8 days and 16 days after silibinin treatment.

RESULTS

In both treatment protocols, silibinin had strong and sustained inhibitory effect on xenograft growth. Detailed xenograft analyses showed that silibinin, in both treatment protocols, exerts anti-proliferative, pro-apoptotic and anti-angiogenic effects. Further, silibinin reduced the expression of β-catenin and phospho-GSK3β in xenograft tissues. Silibinin also targeted signaling molecules involved in CRC proliferation and survival (cyclin D1, c-Myc and survivin) as well as angiogenesis regulators (VEGF and iNOS).

CONCLUSIONS

Collectively, these findings substantiate silibinin's therapeutic efficacy against CRC, advocating its translational potential.

Metformin suppresses azoxymethane-induced colorectal aberrant crypt foci by activating AMP-activated protein kinase

Metformin is widely used for the treatment of diabetes mellitus. Adenosine monophosphate-activated protein kinase (AMPK) is known to be activated by metformin and to inhibit the mammalian target of rapamycin (mTOR) pathway. The mTOR pathway plays an important role in the protein translational machinery and cell proliferation. We examined the effect of metformin on the suppression of colorectal carcinogenesis in chemical carcinogen-induced models. Seven-wk-old BALB/c mice were intraperitoneally (i.p.) injected with azoxymethane (AOM, 10 mg/kg) and then treated with or without metformin (250 mg/kg/d) for 6 wk (for the investigation of aberrant crypt foci [ACF] formation) or 32 wk (for polyp formation). We next investigated colonic epithelial proliferation using bromodeoxyuridine (BrdU) and the proliferating cell nuclear antigen (PCNA) labeling indices. Furthermore, to examine the indirect effect of metformin, the insulin resistance status and the serum lipid levels were assessed. Treatment with metformin significantly reduced ACF formation. The effect of metformin on colon polyp inhibition was relatively modest. No significant difference in body weight or glucose concentration was observed. The BrdU and PCNA indices decreased in mice treated with metformin. A Western blot analysis revealed that the phosphorylated mTOR, S6 kinase, and S6 protein levels in the colonic mucosa decreased significantly in mice treated with metformin. In conclusion, metformin suppresses colonic epithelial proliferation via the inhibition of the mTOR pathway through the activation of AMPK. As metformin is already used daily as an antidiabetic drug, it might be a safe and promising candidate for the chemoprevention of colorectal cancer.

In vitro and in vivo studies on stilbene analogs as potential treatment agents for colon cancer

Based on the potential of resveratrol as a colon cancer chemopreventive agent, a set of 26 stilbenes were synthesized and tested against the colon cancer cell lines HT-29 and Caco-2. (Z)-4-(3,5-Dimethoxystyryl)aniline (4), (Z)-methyl-4-(3,5-dimethoxystyryl)benzoate (6), and (Z)-1,3-dimethoxy-5-(4-methoxystyryl)benzene (10) showed strong inhibitory activity in vitro. In vivo studies using HT-29 xenografts in immunodeficient mice were conducted with 4, 6 and 10, together with their corresponding trans isomers (3, 5, and 9, respectively), at the dose of 10 mg/kg body weight. Tumor volume was significantly lowered in 3-, 4-, and 9-treated groups. The cis- and trans-amino analogs (4 and 3, respectively) had similar effect on tumor growth, a 40% decrease compared to the control. Analysis of the serum revealed that 4 isomerized to 3, which may explain their similar effects in SCID mice. Stilbenes 5, 6, 9, and 10 retained their configurations in the serum. Stilbenes 6 and 10 lacked tumor-suppressive effect in SCID mice; the serum levels of these analogs were low (18.8 and 15.5 ng/mL, respectively). Stilbene 9, while weakly active in vitro demonstrated good activity in vivo, was found at higher levels in the serum (69.9 ng/mL) compared to 10. The anti-tumorigenic activity of these stilbene analogs may be partly linked to their effects on proteins involved in cell proliferation, as observed by lowered expression of proliferating cell nuclear antigen and upregulation of the cyclin-dependent kinase inhibitor, p27, in the tumor tissues. Overall, identification of the anti-tumorigenic potential of these compounds provides opportunities for their use against colorectal cancer.

Binding to PCNA in Euryarchaeal DNA Replication requires two PIP motifs for DNA polymerase D and one PIP motif for DNA polymerase B

Replicative DNA polymerases possess a canonical C-terminal proliferating cell nuclear antigen (PCNA)-binding motif termed the PCNA-interacting protein (PIP) box. We investigated the role of the PIP box on the functional interactions of the two DNA polymerases, PabPol B (family B) and PabPol D (family D), from the hyperthermophilic euryarchaeon Pyrococcus abyssi, with its cognate PCNA. The PIP box was essential for interactions of PabPol B with PCNA, as shown by surface plasmon resonance and primer extension studies. In contrast, binding of PabPol D to PCNA was affected only partially by removing the PIP motif. We identified a second palindromic PIP box motif at the N-terminus of the large subunit of PabPol D that was required for the interactions of PabPol D with PCNA. Thus, two PIP motifs were needed for PabPol D for binding to PabPCNA. Moreover, the C-terminus of PabPCNA was essential for stimulation of PabPol D activity but not for stimulation of PabPol B activity. Neither DNA polymerase interacted with the PabPCNA interdomain connecting loop. Our data suggest that distinct processes are involved in PabPol D and PabPol B binding to PCNA, raising the possibility that Archaea require two mechanisms for recruiting replicative DNA polymerases at the replication fork.

DNA polymerase family X: function, structure, and cellular roles

The X family of DNA polymerases in eukaryotic cells consists of terminal transferase and DNA polymerases beta, lambda, and mu. These enzymes have similar structural portraits, yet different biochemical properties, especially in their interactions with DNA. None of these enzymes possesses a proofreading subdomain, and their intrinsic fidelity of DNA synthesis is much lower than that of a polymerase that functions in cellular DNA replication. In this review, we discuss the similarities and differences of three members of Family X: polymerases beta, lambda, and mu. We focus on biochemical mechanisms, structural variation, fidelity and lesion bypass mechanisms, and cellular roles. Remarkably, although these enzymes have similar three-dimensional structures, their biochemical properties and cellular functions differ in important ways that impact cellular function.

An insight into the biological functions of family X-DNA polymerase in DNA replication and repair of plant genome

Recently we have reported the characterization of a novel single subunit 62-kDa polypeptide with ddNTP-sensitive DNA polymerase activity from the developing seeds of mungbean (Vigna radiata). The protein showed higher expression and activity level during nuclear endoreduplication stages of mungbean seeds and similarity with mammalian DNA polymerase beta in many physicochemical properties. The enzyme was found to specifically interact with PCNA (proliferating cell nuclear antigen), and expressed in both meristematic and meiotic tissues. Functional assays have demonstrated binding of the enzyme to normal and mismatched DNA substrates and with fidelity DNA synthesis in moderately processive mode, suggesting probable involvement of the enzyme in both replication and recombination. Here we have discussed the position of mungbean DNA polymerase as a homologue of DNA Pol lambda, one of the newly identified member of family-X DNA polymerase in plants and illustrated the functional relevance of this enzyme in maintaining the coordination between DNA replication and repair in plant genome.

Inhibition of azoxymethane-induced colonic aberrant crypt foci formation by silibinin in male Fisher 344 rats

Chemoprevention is a practical approach to control colorectal cancer, which is one of the major causes of cancer mortality in the United States. Based on our recent silibinin efficacy studies in human colorectal cancer cells, we investigated the effects of its dietary feeding on azoxymethane (AOM)-induced aberrant crypt foci (ACF) formation and associated biomarkers in male Fisher 344 rats. Five-week-old male Fisher 344 rats were fed control or silibinin-supplemented (0.033%, 0.1%, 0.33%, or 1%, w/w) diet. After 2 weeks, AOM was injected once a week for 2 weeks while silibinin treatments were continued. In another protocol, identical silibinin treatments were done but started 2 weeks post-AOM initiation. All rats were sacrificed at 16 weeks of age, and colon samples were evaluated for ACF, followed by proliferation, apoptosis, and inducible nitric oxide synthase and cyclooxygenase-2, by immunohistochemistry and/or immunoblotting. Silibinin significantly (P < 0.001) reduced dose-dependently the number and multiplicity of AOM-induced ACF formation. Silibinin feeding in pre- and post-AOM initiation decreased mean number of ACF by 39% to 65% and in post-AOM initiation by 29% to 55%. Silibinin dose-dependently decreased AOM-induced colonic cell proliferation, evidenced by proliferative cell nuclear antigen and cyclin D1 immunohistochemical staining, and induced apoptosis in these colon tissues, evidenced by terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling staining and cleaved poly(ADP-ribose) polymerase. Furthermore, silibinin significantly decreased AOM-induced inducible nitric oxide synthase- and cyclooxygenase-2-positive cells in colon tissues. The present findings show possible beneficial activity of silibinin at least in early stage of colon tumorigenesis, suggesting that silibinin might be an effective natural agent for colorectal cancer chemoprevention.

Interaction between DNA Polymerase lambda and RPA during translesion synthesis

Replication of damaged DNA (translesion synthesis, TLS) is realized by specialized DNA polymerases. Additional protein factors such as replication protein A (RPA) play important roles in this process. However, details of the interaction are unknown. Here we analyzed the influence of the hRPA and its mutant hABCD lacking domains responsible for protein-protein interactions on ability of DNA polymerase lambda to catalyze TLS. The primer-template structures containing varying parts of extended strand (16 and 37 nt) were used as model systems imitating DNA intermediate of first stage of TLS. The 8-oxoguanine disposed in +1 position of the template strand in relation to 3 -end of primer was exploited as damage. It was shown that RPA stimulated TLS DNA synthesis catalyzed by DNA polymerase lambda in its globular but not in extended conformation. Moreover, this effect is dependent on the presence of p70N and p32C domains in RPA molecule.

Analysis of processivity of mungbean dideoxynucleotide-sensitive DNA polymerase and detection of the activity and expression of the enzyme in the meristematic and meiotic tissues and following DNA damaging agent

Analysis of the processivity of mungbean ddNTP-sensitive DNA polymerase showed the incorporation of approximately 35-40 nucleotides per binding event in the replication assays involving M13 ss DNA template with 5'-labeled 17-mer primer. Optimal processivity was obtained with 100-150 mM KCl and 6-8 mM Mg2+ at pH 7.5. The enzyme showed preference for Mg2+ over Mn2+ as the metal activator for processivity. 2', 3' dideoxythymidine 5' triphosphate (ddTTP) and rat DNA pol beta antibody strongly influenced distributive synthesis. Considerable enhancement in processivity was noticed at 1mM ATP and 2-4 mM spermidine while higher concentrations of spermidine caused distributive synthesis. The enzyme was found to be active in both meristematic and meiotic tissues and distinctly induced by EMS treatment. DNA-binding assays revealed distinct binding ability of the enzyme to template/primer and damaged DNA substrate. Together these observations illustrate the probable involvement of the enzyme in replication and repair machinery in higher plants.

DNA polymerases and human diseases

DNA polymerases function in DNA replication, repair, recombination and translesion synthesis. Currently, 15 DNA polymerase genes have been identified in human cells, belonging to four distinct families. In this review, we briefly describe the biochemical activities and known cellular roles of each DNA polymerase. Our major focus is on the phenotypic consequences of mutation or ablation of individual DNA polymerase genes. We discuss phenotypes of current mouse models and altered polymerase functions and the relationship of DNA polymerase gene mutations to human cell phenotypes. Interestingly, over 120 single nucleotide polymorphisms (SNPs) have been identified in human populations that are predicted to result in nonsynonymous amino acid substitutions of DNA polymerases. We discuss the putative functional consequences of these SNPs in relation to human disease.

Direct binding of TReP-132 with TdT results in reduction of TdT activity

N regions at the junction of V, D and J DNA segments are synthesized with large protein complexes including terminal deoxynucleotidyltransferase (TdT) during V(D)J recombination in B- or T-cells. TdT directly binds to TdIF1, TdIF2, PCNA and the Ku70/86 heterodimer. Using a yeast two-hybrid system, we isolated a cDNA clone encoding the gene for TReP-132, which is involved in P450scc gene expression in steroid-hormone-producing cells or lymphoid cells. Interaction between TReP-132 and TdIF1 was confirmed by pull-down assay and immunoprecipitation assay using specific antibodies against TReP-132 both in vitro and in vivo. TdT also directly bound to TReP-132 through its confined N-terminal region. Furthermore, the co-expression of TdIF1 and TReP-132 or TdT and TReP-132 in COS7 cells showed that these proteins are co-localized within the nucleus. TReP-132 reduces TdT activity to 2.5% of its maximum value in the in vitro assay system using double-stranded DNA with a 3' protrusion as a primer. These findings suggest that TdT synthesizes N region under a negative control of TReP-132 during V(D)J recombination.

Structure–function studies of DNA polymerase lambda

DNA polymerase lambda is a member of the X family of polymerases that is implicated in non-homologous end-joining of double-strand breaks in DNA and in base excision repair of DNA damage. To better understand the roles of DNA polymerase lambda in these repair pathways, here we review its structure and biochemical properties, with emphasis on its gap-filling polymerization activity, its dRP lyase activity and its unusual DNA synthetic (in)fidelity.

Phosphorylation of human DNA polymerase lambda by the cyclin-dependent kinase Cdk2/cyclin A complex is modulated by its association with proliferating cell nuclear antigen

DNA polymerase (Pol) lambda is a member of the Pol X family and possesses four different enzymatic activities, being DNA polymerase, terminal transferase, deoxyribose phosphate lyase and polynucleotide synthetase, all localized in its C-terminal region. On the basis of its biochemical properties, Pol lambda has been implicated in various DNA repair pathways, such as abasic site translesion DNA synthesis, base excision repair and non-homologous end joining of double strand breaks. However, its role in vivo has not yet been elucidated. In addition, Pol lambda has been shown to interact with the replication clamp proliferating cell nuclear antigen (PCNA) in vitro and in vivo. In this work, we searched by affinity chromatography for novel partners and we identified the cyclin-dependent kinase Cdk2 as novel partner of Pol lambda. Pol lambda is phosphorylated in vitro by several Cdk/cyclin complexes, including Cdk2/cyclin A, in its proline-serine-rich domain. While the polymerase activity of Pol lambda was not affected by Cdk2/cyclin A phosphorylation, phosphorylation of Pol lambda was decreased by its interaction with PCNA. Finally, Pol lambda is also phosphorylated in vivo in human cells and this phosphorylation is modulated during the cell cycle.