CD271 mRNA/hnRNPA2B1 complex promotes proliferation and stemness in oral and head and neck squamous cell carcinoma.

RNAs, such as noncoding RNA, microRNA, and recently mRNA, have been recognized as signal transduction molecules. CD271, also known as nerve growth factor receptor, has a critical role in cancer, although the precise mechanism is still unclear. Here, we show that CD271 mRNA, but not CD271 protein, facilitates spheroid cell proliferation. We established CD271-/- cells lacking both mRNA and protein of CD271, as well as CD271 protein knockout cells lacking only CD271 protein, from hypopharyngeal and oral squamous cell carcinoma lines. Sphere formation was reduced in CD271-/- cells but not in CD271 protein knockout cells. Mutated CD271 mRNA, which is not translated to a protein, promoted sphere formation. CD271 mRNA bound to hnRNPA2B1 protein at the 3'-UTR region, and the inhibition of this interaction reduced sphere formation. In surgical specimens, the CD271 mRNA/protein expression ratio was higher in the cancerous area than in the noncancerous area. These data suggest CD271 mRNA has dual functions, encompassing protein-coding and noncoding roles, with its noncoding RNA function being predominant in oral and head and neck squamous cell carcinoma.

FAXC interacts with ANXA2 and SRC in mitochondria and promotes tumorigenesis in cholangiocarcinoma.

Cholangiocarcinoma (CCA) is one of the most difficult malignancies to treat as the therapeutic options are limited. Although several driver genes have been identified, most remain unknown. In this study, we identified a failed axon connection homolog (FAXC), whose function is unknown in mammals, by analyzing serially passaged CCA xenograft models. Knockdown of FAXC reduced subcutaneous tumorigenicity in mice. FAXC was bound to annexin A2 (ANXA2) and c-SRC, which are tumor-promoting genes. The FAXC/ANXA2/c-SRC complex forms in the mitochondria. FAXC enhances SRC-dependent ANXA2 phosphorylation at tyrosine-24, and the C-terminal amino acid residues (351-375) of FAXC are required for ANXA2 phosphorylation. Transcriptome data from a xenografted CCA cell line revealed that FAXC correlated with epithelial-mesenchymal transition, hypoxia, and KRAS signaling genes. Collectively, these findings advance our understanding of CCA tumorigenesis and provide candidate therapeutic targets.

The BRCA1/BARD1-interacting protein OLA1 functions in centrosome regulation.

The breast and ovarian cancer-specific tumor suppressor BRCA1, along with its heterodimer partner BRCA1-associated RING domain protein (BARD1), plays important roles in DNA repair, centrosome regulation, and transcription. To explore further functions of BRCA1/BARD1, we performed mass spectrometry analysis and identified Obg-like ATPase 1 (OLA1) as a protein that interacts with the carboxy-terminal region of BARD1. OLA1 directly bound to the amino-terminal region of BRCA1 and γ-tubulin. OLA1 localized to centrosomes in interphase and to the spindle pole in mitotic phase, and its knockdown resulted in centrosome amplification and the activation of microtubule aster formation. OLA1 with a mutation observed in breast cancer cell line, E168Q, failed to bind BRCA1 and rescue the OLA1 knockdown-induced centrosome amplification. BRCA1 variant I42V also abrogated the binding of BRCA1 to OLA1. These findings suggest that OLA1 plays an important role in centrosome regulation together with BRCA1.

Loss of Axdnd1 causes sterility due to impaired spermatid differentiation in mice.

Purpose: Spermiogenesis, the process of deformation of sperm head morphology and flagella formation, is a phenomenon unique to sperm. Axonemal dynein light chain proteins are localized to sperm flagella and are known to be involved in sperm motility. Here, we focused on the gene axonemal dynein light chain domain containing 1 (Axdnd1) with the aim to determine the function of its protein product AXDND1. Methods: To elucidate the role of AXDND1 in spermatogenesis, we generated Axdnd1 knockout (KO) mice using the CRISPR/Cas9 system. The generated mice were subjected to fertility tests and analyzed by immunohistochemistry. Result: The Axdnd1 KO mouse exhibited sterility caused by impaired spermiogenesis during the elongation step as well as abnormal nuclear shaping and manchette, which are essential for spermiogenesis. Moreover, AXDND1 showed enriched testicular expression and was localized from the mid-pachytene spermatocytes to the early spermatids. Conclusion: Axdnd1 is essential for spermatogenesis in the mouse testes. These findings improve our understanding of spermiogenesis and related defects. According to a recent report, deleterious heterozygous mutations in AXDND1 were found in non-obstructive azoospermia (NOA) patients. Therefore, Axdnd1 KO mice could be used as a model system for NOA, which will greatly contribute to future NOA treatment studies.

AMPK Complex Activation Promotes Sarcolemmal Repair in Dysferlinopathy.

Mutations in dysferlin are responsible for a group of progressive, recessively inherited muscular dystrophies known as dysferlinopathies. Using recombinant proteins and affinity purification methods combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS), we found that AMP-activated protein kinase (AMPK)γ1 was bound to a region of dysferlin located between the third and fourth C2 domains. Using ex vivo laser injury experiments, we demonstrated that the AMPK complex was vital for the sarcolemmal damage repair of skeletal muscle fibers. Injury-induced AMPK complex accumulation was dependent on the presence of Ca2+, and the rate of accumulation was regulated by dysferlin. Furthermore, it was found that the phosphorylation of AMPKα was essential for plasma membrane repair, and treatment with an AMPK activator rescued the membrane-repair impairment observed in immortalized human myotubes with reduced expression of dysferlin and dysferlin-null mouse fibers. Finally, it was determined that treatment with the AMPK activator metformin improved the muscle phenotype in zebrafish and mouse models of dysferlin deficiency. These findings indicate that the AMPK complex is essential for plasma membrane repair and is a potential therapeutic target for dysferlinopathy.

Identification of Binding Proteins for TSC22D1 Family Proteins Using Mass Spectrometry.

TSC-22 (TGF-ß stimulated clone-22) has been reported to induce differentiation, growth inhibition, and apoptosis in various cells. TSC-22 is a member of a family in which many proteins are produced from four different family genes. TSC-22 (corresponding to TSC22D1-2) is composed of 144 amino acids translated from a short variant mRNA of the TSC22D1 gene. In this study, we attempted to determine the intracellular localizations of the TSC22D1 family proteins (TSC22D1-1, TSC-22 (TSC22D1-2), and TSC22(86) (TSC22D1-3)) and identify the binding proteins for TSC22D1 family proteins by mass spectrometry. We determined that TSC22D1-1 was mostly localized in the nucleus, TSC-22 (TSC22D1-2) was localized in the cytoplasm, mainly in the mitochondria and translocated from the cytoplasm to the nucleus after DNA damage, and TSC22(86) (TSC22D1-3) was localized in both the cytoplasm and nucleus. We identified multiple candidates of binding proteins for TSC22D1 family proteins in in vitro pull-down assays and in vivo binding assays. Histone H1 bound to TSC-22 (TSC22D1-2) or TSC22(86) (TSC22D1-3) in the nucleus. Guanine nucleotide-binding protein-like 3 (GNL3), which is also known as nucleostemin, bound to TSC-22 (TSC22D1-2) in the nucleus. Further investigation of the interaction of the candidate binding proteins with TSC22D1 family proteins would clarify the biological roles of TSC22D1 family proteins in several cell systems.

Cep128 associates with Odf2 to form the subdistal appendage of the centriole.

The mother centriole in a cell has two appendages, the distal appendage (DA) and subdistal appendage (SDA), which have roles in generating cilia and organizing the cellular microtubular network, respectively. In the knockout (KO) cells of Odf2, the component of the DA and SDA, both appendages simultaneously disappear. However, the molecular mechanisms by which the DA and SDA form independently but close to each other downstream of Odf2 are unknown. Here, using super-resolution structured illumination microscopy (SR-SIM), we found that the signal for GFP-tagged Odf2 overlapped considerably with that of immunofluorescently labeled Cep128. We further found that Cep128 knockdown (KD) caused the dissociation of other SDA components from the centriole, including centriolin, Ndel1, ninein and Cep170, whereas Odf2 was still associated with the centriole. In contrast, the DA components remained associated with the centriole in Cep128 KD cells. Consistent with this observation, we identified Cep128 as an Odf2-interacting protein by immunoprecipitation. Taken with the finding that Cep128 deletion decreased the stability of centriolar microtubules, our results indicate that Cep128 associates with Odf2 in the hierarchical assembly of SDA components to elicit the microtubule-organizing function.

Delineation of LZTR1 mutation-positive patients with Noonan syndrome and identification of LZTR1 binding to RAF1-PPP1CB complexes.

RASopathies are a group of developmental disorders caused by mutations in genes that regulate the RAS/MAPK pathway and include Noonan syndrome (NS), Costello syndrome, cardiofaciocutaneous syndrome and other related disorders. Whole exome sequencing studies recently identified LZTR1, PPP1CB and MRAS as new causative genes in RASopathies. However, information on the phenotypes of LZTR1 mutation-positive patients and functional properties of the mutations are limited. To identify variants of LZTR1, PPP1CB, and MRAS, we performed a targeted next-generation sequencing and reexamined previously analyzed exome data in 166 patients with suspected RASopathies. We identified eight LZTR1 variants, including a de novo variant, in seven probands who were suspicious for NS and one known de novo PPP1CB variant in a patient with NS. One of the seven probands had two compound heterozygous LZTR1 variants, suggesting autosomal recessive inheritance. All probands with LZTR1 variants had cardiac defects, including hypertrophic cardiomyopathy and atrial septal defect. Five of the seven probands had short stature or intellectual disabilities. Immunoprecipitation of endogenous LZTR1 followed by western blotting showed that LZTR1 bound to the RAF1-PPP1CB complex. Cells transfected with a small interfering RNA against LZTR1 exhibited decreased levels of RAF1 phosphorylated at Ser259. These are the first results to demonstrate LZTR1 in association with the RAF1-PPP1CB complex as a component of the RAS/MAPK pathway.

Dynamic feature of mitotic arrest deficient 2-like protein 2 (MAD2L2) and structural basis for its interaction with chromosome alignment-maintaining phosphoprotein (CAMP).

Mitotic arrest deficient 2-like protein 2 (MAD2L2), also termed MAD2B or REV7, is involved in multiple cellular functions including translesion DNA synthesis (TLS), signal transduction, transcription, and mitotic events. MAD2L2 interacts with chromosome alignment-maintaining phosphoprotein (CAMP), a kinetochore-microtubule attachment protein in mitotic cells, presumably through a novel "WK" motif in CAMP. Structures of MAD2L2 in complex with binding regions of the TLS proteins REV3 and REV1 have revealed that MAD2L2 has two faces for protein-protein interactions that are regulated by its C-terminal region; however, the mechanisms underlying the MAD2L2-CAMP interaction and the mitotic role of MAD2L2 remain unknown. Here we have determined the structures of human MAD2L2 in complex with a CAMP fragment in two crystal forms. The overall structure of the MAD2L2-CAMP complex in both crystal forms was essentially similar to that of the MAD2L2-REV3 complex. However, the residue interactions between MAD2L2 and CAMP were strikingly different from those in the MAD2L2-REV3 complex. Furthermore, structure-based interaction analyses revealed an unprecedented mechanism involving CAMP's WK motif. Surprisingly, in one of the crystal forms, the MAD2L2-CAMP complex formed a dimeric structure in which the C-terminal region of MAD2L2 was swapped and adopted an immature structure. The structure provides direct evidence for the dynamic nature of MAD2L2 structure, which in turn may have implications for the protein-protein interaction mechanism and the multiple functions of this protein. This work is the first structural study of MAD2L2 aside from its role in TLS and might pave the way to clarify MAD2L2's function in mitosis.

The ACF1 complex is required for DNA double-strand break repair in human cells.

DNA double-strand breaks (DSBs) are repaired via nonhomologous end-joining (NHEJ) or homologous recombination (HR), but cellular repair processes remain elusive. We show here that the ATP-dependent chromatin-remodeling factors, ACF1 and SNF2H, accumulate rapidly at DSBs and are required for DSB repair in human cells. If the expression of ACF1 or SNF2H is suppressed, cells become extremely sensitive to X-rays and chemical treatments producing DSBs, and DSBs remain unrepaired. ACF1 interacts directly with KU70 and is required for the accumulation of KU proteins at DSBs. The KU70/80 complex becomes physically more associated with the chromatin-remodeling factors of the CHRAC complex, which includes ACF1, SNF2H, CHRAC15, and CHRAC17, after treatments producing DSBs. Furthermore, the frequency of NHEJ as well as HR induced by DSBs in chromosomal DNA is significantly decreased in cells depleted of either of these factors. Thus, ACF1 and its complexes play important roles in DSBs repair.

A pleckstrin homology-like domain is critical for F-actin binding and cofilin-phosphatase activity of Slingshot-1.

Slingshot-1 (SSH1) is a protein phosphatase that specifically dephosphorylates and activates cofilin, an F-actin-severing protein. SSH1 binds to and co-localizes with F-actin, and the cofilin-phosphatase activity of SSH1 is markedly increased by binding to F-actin. In this study, we performed a secondary structure analysis of SSH1, which predicted the existence of a pleckstrin homology (PH)-like domain in the N-terminal region of SSH1. SSH1 also contains a DEK-C domain in the N-terminal region. The N-terminal fragment of SSH1 bound to and co-localized with F-actin, but mutation at Arg-96 or a Leu-His-Lys (LHK) motif in the PH-like domain reduced this activity. Furthermore, mutation at Arg-96 abrogated the cofilin-phosphatase activity of SSH1 in the presence of F-actin. These results suggest that the N-terminal PH-like domain plays a critical role in F-actin binding and F-actin-mediated activation of the cofilin-phosphatase activity of SSH1.

Mitochonic Acid 5 Binds Mitochondria and Ameliorates Renal Tubular and Cardiac Myocyte Damage.

Mitochondrial dysfunction causes increased oxidative stress and depletion of ATP, which are involved in the etiology of a variety of renal diseases, such as CKD, AKI, and steroid-resistant nephrotic syndrome. Antioxidant therapies are being investigated, but clinical outcomes have yet to be determined. Recently, we reported that a newly synthesized indole derivative, mitochonic acid 5 (MA-5), increases cellular ATP level and survival of fibroblasts from patients with mitochondrial disease. MA-5 modulates mitochondrial ATP synthesis independently of oxidative phosphorylation and the electron transport chain. Here, we further investigated the mechanism of action for MA-5. Administration of MA-5 to an ischemia-reperfusion injury model and a cisplatin-induced nephropathy model improved renal function. In in vitro bioenergetic studies, MA-5 facilitated ATP production and reduced the level of mitochondrial reactive oxygen species (ROS) without affecting activity of mitochondrial complexes I-IV. Additional assays revealed that MA-5 targets the mitochondrial protein mitofilin at the crista junction of the inner membrane. In Hep3B cells, overexpression of mitofilin increased the basal ATP level, and treatment with MA-5 amplified this effect. In a unique mitochondrial disease model (Mitomice with mitochondrial DNA deletion that mimics typical human mitochondrial disease phenotype), MA-5 improved the reduced cardiac and renal mitochondrial respiration and seemed to prolong survival, although statistical analysis of survival times could not be conducted. These results suggest that MA-5 functions in a manner differing from that of antioxidant therapy and could be a novel therapeutic drug for the treatment of cardiac and renal diseases associated with mitochondrial dysfunction.

Insulin receptor substrate-4 binds to Slingshot-1 phosphatase and promotes cofilin dephosphorylation.

Cofilin plays an essential role in cell migration and morphogenesis by enhancing actin filament dynamics via its actin filament-severing activity. Slingshot-1 (SSH1) is a protein phosphatase that plays a crucial role in regulating actin dynamics by dephosphorylating and reactivating cofilin. In this study, we identified insulin receptor substrate (IRS)-4 as a novel SSH1-binding protein. Co-precipitation assays revealed the direct endogenous binding of IRS4 to SSH1. IRS4, but not IRS1 or IRS2, was bound to SSH1. IRS4 was bound to SSH1 mainly through the unique region (amino acids 335-400) adjacent to the C terminus of the phosphotyrosine-binding domain of IRS4. The N-terminal A, B, and phosphatase domains of SSH1 were bound to IRS4 independently. Whereas in vitro phosphatase assays revealed that IRS4 does not directly affect the cofilin phosphatase activity of SSH1, knockdown of IRS4 increased cofilin phosphorylation in cultured cells. Knockdown of IRS4 decreased phosphatidylinositol 3-kinase (PI3K) activity, and treatment with an inhibitor of PI3K increased cofilin phosphorylation. Akt preferentially phosphorylated SSH1 at Thr-826, but expression of a non-phosphorylatable T826A mutant of SSH1 did not affect insulin-induced cofilin dephosphorylation, and an inhibitor of Akt did not increase cofilin phosphorylation. These results suggest that IRS4 promotes cofilin dephosphorylation through sequential activation of PI3K and SSH1 but not through Akt. In addition, IRS4 co-localized with SSH1 in F-actin-rich membrane protrusions in insulin-stimulated cells, which suggests that the association of IRS4 with SSH1 contributes to localized activation of cofilin in membrane protrusions.

CAMP (C13orf8, ZNF828) is a novel regulator of kinetochore-microtubule attachment.

Proper attachment of microtubules to kinetochores is essential for accurate chromosome segregation. Here, we report a novel protein involved in kinetochore-microtubule attachment, chromosome alignment-maintaining phosphoprotein (CAMP) (C13orf8, ZNF828). CAMP is a zinc-finger protein containing three characteristic repeat motifs termed the WK, SPE, and FPE motifs. CAMP localizes to chromosomes and the spindle including kinetochores, and undergoes CDK1-dependent phosphorylation at multiple sites during mitosis. CAMP-depleted cells showed severe chromosome misalignment, which was associated with the poor resistance of K-fibres to the tension exerted upon establishment of sister kinetochore bi-orientation. We found that the FPE region, which is responsible for spindle and kinetochore localization, is essential for proper chromosome alignment. The C-terminal region containing the zinc-finger domains negatively regulates chromosome alignment, and phosphorylation in the FPE region counteracts this regulation. Kinetochore localization of CENP-E and CENP-F was affected by CAMP depletion, and by expressing CAMP mutants that cannot functionally rescue CAMP depletion, placing CENP-E and CENP-F as downstream effectors of CAMP. These data suggest that CAMP is required for maintaining kinetochore-microtubule attachment during bi-orientation.

Furry promotes acetylation of microtubules in the mitotic spindle by inhibition of SIRT2 tubulin deacetylase.

The structure and function of microtubules (MTs) are regulated by post-translational modifications of tubulin subunits, such as acetylation of the Lys40 residue of α-tubulin. Regulation of the organization and dynamics of MTs is essential for the precise formation of the mitotic spindle. Spindle MTs are highly acetylated, but the mechanism regulating this acetylation is largely unknown. Furry (Fry) is an evolutionarily conserved protein that binds to MTs and colocalizes with acetylated MTs in the mitotic spindle. In this study, we examined the role of Fry in the acetylation of MTs in the mitotic spindle. Depletion of Fry significantly reduced the level of MT acetylation in the mitotic spindle. Expression of the N-terminal fragment of Fry induced hyperacetylation of MTs in both mitotic and interphase cells. These results indicate that Fry promotes MT acetylation in the mitotic spindle. We also found that Fry binds to the tubulin deacetylase SIRT2, preferentially in mitotic cells. Cell-free experiments revealed that the N-terminal region of Fry is the domain responsible for binding to and inhibiting the tubulin-deacetylase activity of SIRT2. AGK2, a specific inhibitor of SIRT2, increased the level of MT acetylation in the mitotic spindle, indicating that SIRT2 is involved in the deacetylation of spindle MTs. Furthermore, AGK2 reversed the decrease in MT acetylation induced by Fry depletion. In summary, these results suggest that Fry plays a crucial role in promoting the level of MT acetylation in the mitotic spindle by inhibiting the tubulin-deacetylase activity of SIRT2.

Activation of cytosolic Slingshot-1 phosphatase by gelsolin-generated soluble actin filaments.

Slingshot-1 (SSH1) is a protein phosphatase that dephosphorylates and activates cofilin, an actin-severing and -disassembling protein. SSH1 is bound to and activated by F-actin, but not G-actin. SSH1 is accumulated in the F-actin-rich lamellipodium but is also diffusely distributed in the cytoplasm. It remains unknown whether SSH1 is activated by soluble (low-level polymerized) actin filaments in the cytoplasm. In this study, we show that SSH1 binds to gelsolin via actin filaments in the cytosolic fraction. Gelsolin promoted solubilization of actin filaments and SSH1 in cell-free assays and in cultured cells. SSH1 was activated by gelsolin-generated soluble actin filaments. Furthermore, gelsolin enhanced cofilin dephosphorylation in neuregulin-stimulated cells. Our results suggest that cytosolic SSH1 forms a complex with gelsolin via soluble actin filaments and is activated by gelsolin-generated soluble actin filaments and that gelsolin promotes stimulus-induced cofilin dephosphorylation through increasing soluble actin filaments, which support SSH1 activation in the cytoplasm.

Nucleoporin Nup188 is required for chromosome alignment in mitosis.

Most cancer cells are aneuploid, which could be caused by defects in chromosome segregation machinery. Nucleoporins (Nup) are components of the nuclear pore complex, which is essential for nuclear transport during interphase, but several nucleoporins are also known to be involved in chromosome segregation. Here we report a novel function of Nup188, one of the nucleoporins regulating chromosome segregation. Nup188 localizes to spindle poles during mitosis, through the C-terminal region of Nup188. In Nup188-depleted mitotic cells, chromosomes fail to align to the metaphase plate, which causes mitotic arrest due to the spindle assembly checkpoint. Both the middle and the C-terminal regions were required for chromosome alignment. Robust K-fibers, microtubule bundles attaching to kinetochores, were hardly formed in Nup188-depleted cells. Significantly, we found that Nup188 interacts with NuMA, which plays an instrumental role in focusing microtubules at centrosomes, and NuMA localization to spindle poles is perturbed in Nup188-depleted cells. These data suggest that Nup188 promotes chromosome alignment through K-fiber formation and recruitment of NuMA to spindle poles.

LZTR1 deficiency exerts high metastatic potential by enhancing sensitivity to EMT induction and controlling KLHL12-mediated collagen secretion.

Leucine zipper-like transcriptional regulator 1 (LZTR1), a substrate adaptor of Cullin 3 (CUL3)-based E3 ubiquitin ligase, regulates proteostasis of the RAS subfamily. Mutations in LZTR1 have been identified in patients with several types of cancer. However, the role of LZTR1 in tumor metastasis and the target molecules of LZTR1, excluding the RAS subfamily, are not clearly understood. Here, we show that LZTR1 deficiency increases tumor growth and metastasis. In lung adenocarcinoma cells, LZTR1 deficiency induced the accumulation of the RAS subfamily and enhanced cell proliferation, invasion, and xenograft tumor growth. Multi-omics analysis to clarify the pathways related to tumor progression showed that MAPK signaling, epithelial-mesenchymal transition (EMT), and extracellular matrix (ECM) remodeling-related gene ontology terms were enriched in LZTR1 knockout cells. Indeed, LZTR1 deficiency induced high expression of EMT markers under TGF-ß1 treatment. Our search for novel substrates that interact with LZTR1 resulted in the discovery of a Kelch-like protein 12 (KLHL12), which is involved in collagen secretion. LZTR1 could inhibit KLHL12-mediated ubiquitination of SEC31A, a component of coat protein complex II (COPII), whereas LZTR1 deficiency promoted collagen secretion. LZTR1-RIT1 and LZTR1-KLHL12 worked independently regarding molecular interactions and did not directly interfere with each other. Further, we found that LZTR1 deficiency significantly increases lung metastasis and promotes ECM deposition around metastatic tumors. Since collagen-rich extracellular matrix act as pathways for migration and facilitate metastasis, increased expression of RAS and collagen deposition may exert synergistic or additive effects leading to tumor progression and metastasis. In conclusion, LZTR1 deficiency exerts high metastatic potential by enhancing sensitivity to EMT induction and promoting collagen secretion. The functional inhibition of KLHL12 by LZTR1 provides important evidence that LZTR1 may be a repressor of BTB-Kelch family members. These results provide clues to the mechanism of LZTR1-deficiency carcinogenesis.

Polynucleotide kinase and aprataxin-like forkhead-associated protein (PALF) acts as both a single-stranded DNA endonuclease and a single-stranded DNA 3' exonuclease and can participate in DNA end joining in a biochemical system.

Polynucleotide kinase and aprataxin-like forkhead-associated protein (PALF, also called aprataxin- and PNK-like factor (APLF)) has been shown to have nuclease activity and to use its forkhead-associated domain to bind to x-ray repair complementing defective repair in Chinese hamster cells 4 (XRCC4). Because XRCC4 is a key component of the ligase IV complex that is central to the nonhomologous DNA end joining (NHEJ) pathway, this raises the possibility that PALF might play a role in NHEJ. For this reason, we further studied the nucleolytic properties of PALF, and we searched for any modulation of PALF by NHEJ components. We verified that PALF has 3' exonuclease activity. However, PALF also possesses single-stranded DNA endonuclease activity. This single-stranded DNA endonuclease activity can act at all single-stranded sites except those within four nucleotides 3' of a double-stranded DNA junction, suggesting that PALF minimally requires approximately four nucleotides of single-strandedness. Ku, DNA-dependent protein kinase catalytic subunit, and XRCC4-DNA ligase IV do not modulate PALF nuclease activity on single-stranded DNA or overhangs of duplex substrates. PALF does not open DNA hairpins. However, in a reconstituted end joining assay that includes Ku, XRCC4-DNA ligase IV, and PALF, PALF is able to resect 3' overhanging nucleotides and permit XRCC4-DNA ligase IV to complete the joining process in a manner that is as efficient as Artemis. Reduction of PALF in vivo reduces the joining of incompatible DNA ends. Hence, PALF can function in concert with other NHEJ proteins.

WRNIP1 accumulates at laser light irradiated sites rapidly via its ubiquitin-binding zinc finger domain and independently from its ATPase domain.

WRNIP1 (Werner helicase-interacting protein 1) was originally identified as a protein that interacts with the Werner syndrome responsible gene product. WRNIP1 contains a ubiquitin-binding zinc-finger (UBZ) domain in the N-terminal region and two leucine zipper motifs in the C-terminal region. In addition, it possesses an ATPase domain in the middle of the molecule and the lysine residues serving as ubiquitin acceptors in the entire of the molecule. Here, we report that WRNIP1 accumulates in laser light irradiated sites very rapidly via its ubiquitin-binding zinc finger domain, which is known to bind polyubiquitin and to be involved in ubiquitination of WRNIP1 itself. The accumulation of WRNIP1 in laser light irradiated sites also required the C-terminal region containing two leucine zippers, which is reportedly involved in the oligomerization of WRNIP1. Mutated WRNIP1 with a deleted ATPase domain or with mutations in lysine residues, which serve as ubiquitin acceptors, accumulated in laser light irradiated sites, suggesting that the ATPase domain of WRNIP1 and ubiquitination of WRNIP1 are dispensable for the accumulation.