Depletion of hCINAP by RNA interference causes defects in Cajal body formation, histone transcription, and cell viability

Knockdown of hCINAP sensitizes colorectal cancer cells to ionizing radiation

Colorectal cancer (CRC) poses a significant challenge in terms of treatment due to the prevalence of radiotherapy resistance. However, the underlying mechanisms responsible for radio-resistance in CRC have not been thoroughly explored. This study aimed to shed light on the role of human coilin interacting nuclear ATPase protein (hCINAP) in radiation-resistant HT-29 and SW480 CRC cells (HT-29-IR and SW480-IR) and investigate its potential implications. Firstly, radiation-resistant CRC cell lines were established by subjecting HT-29 and SW480 cells to sequential radiation exposure. Subsequent analysis revealed a notable increase in hCINAP expression in radiation-resistant CRC cells. To elucidate the functional role of hCINAP in radio-resistance, knockdown experiments were conducted. Remarkably, knockdown of hCINAP resulted in an elevation of reactive oxygen species (ROS) generation upon radiation treatment and subsequent activation of apoptosis mediated by mitochondria. These observations indicate that hCINAP depletion enhances the radiosensitivity of CRC cells. Conversely, when hCINAP was overexpressed, it was found to enhance the radio-resistance of CRC cells. This suggests that elevated hCINAP expression contributes to the development of radio-resistance. Further investigation revealed an interaction between hCINAP and ATPase family AAA domain containing 3A (ATAD3A). Importantly, ATAD3A was identified as an essential factor in hCINAP-mediated radio-resistance. These findings establish the involvement of hCINAP and its interaction with ATAD3A in the regulation of radio-resistance in CRC cells. Overall, the results of this study demonstrate that upregulating hCINAP expression may improve the survival of radiation-exposed CRC cells. Understanding the intricate molecular mechanisms underlying hCINAP function holds promise for potential strategies in targeted radiation therapy for CRC. These findings emphasize the importance of further research to gain a comprehensive understanding of hCINAP's precise molecular mechanisms and explore its potential as a therapeutic target in overcoming radio-resistance in CRC. By unraveling the complexities of hCINAP and its interactions, novel therapeutic approaches may be developed to enhance the efficacy of radiation therapy and improve outcomes for CRC patients.

hCINAP alleviates senescence by regulating MDM2 via p14ARF and the HDAC1/CoREST complex

Cellular senescence is a major process affected by multiple signals and coordinated by a complex signal response network. Identification of novel regulators of cellular senescence and elucidation of their molecular mechanisms will aid in the discovery of new treatment strategies for aging-related diseases. In the present study, we identified human coilin-interacting nuclear ATPase protein (hCINAP) as a negative regulator of aging. Depletion of cCINAP significantly shortened the lifespan of Caenorhabditis elegans and accelerated primary cell aging. Moreover, mCINAP deletion markedly promoted organismal aging and stimulated senescence-associated secretory phenotype in the skeletal muscle and liver from mouse models of radiation-induced senescence. Mechanistically, hCINAP functions through regulating MDM2 status by distinct mechanisms. On the one hand, hCINAP decreases p53 stability by attenuating the interaction between p14ARF and MDM2; on the other hand, hCINAP promotes MDM2 transcription via inhibiting the deacetylation of H3K9ac in the MDM2 promoter by hindering the HDAC1/CoREST complex integrity. Collectively, our data demonstrate that hCINAP is a negative regulator of aging and provide insight into the molecular mechanisms underlying the aging process.

Unique structural features of the adenylate kinase hCINAP/AK6 and its multifaceted functions in carcinogenesis and tumor progression

Human coilin-interacting nuclear ATPase protein (hCINAP), also known as adenylate kinase 6 (AK6), is an atypical adenylate kinase with critical roles in many biological processes, including gene transcription, ribosome synthesis, cell metabolism, cell proliferation and apoptosis, DNA damage responses, and genome stability. Furthermore, hCINAP/AK6 dysfunction is associated with cancer and various inflammatory diseases. In this review, we summarize the structural features and biological roles of hCINAP in several important signaling pathways, as well as its connection with tumor onset and progression.

hCINAP is potentially a direct target gene of HIF-1 and is required for hypoxia-induced EMT and apoptosis in cervical cancer cells

The early metastasis of cervical cancer is a multistep process requiring the cancer cells to adapt to the signal input from different tissue environments, including hypoxia. Hypoxia-induced epithelial-to-mesenchymal transition (EMT) plays a critical role in the ability to invade surrounding tissues. However, the molecular mechanisms underlying EMT in cervical cancer remain to be elucidated. Herein, we show that hypoxia-inducible factor-1alpha (HIF-1α) and aryl hydrocarbon receptor nuclear translocator (ARNT) are recruited to the human coilin-interacting nuclear ATPase protein (hCINAP) promoter and initiate hCINAP expression in hypoxia. Ablation of hCINAP decreased the migratory capacity and EMT of cervical cancer cells under hypoxic conditions. Furthermore, hCINAP regulated EMT through the Akt–mTOR signaling pathway, and inhibits hypoxia-induced p53-dependent apoptosis. Our data collectively show that hCINAP may have essential roles in the metastasis of cervical cancer and could be a potential target for curing cervical cancer.

Human adenylate kinase 6 regulates WNK1 (with no lysine kinase-1) phosphorylation states and affects ion homeostasis in NT2 cells

Adenylate kinase 6 (AK6), a nucleus localized phosphotransferase in mammalians, shows ubiquitously expression and broad substrate activity in different tissues and cell types. Although the function of AK6 has been extensively studied in different cancer cell lines, its role in mammalian germline is still unknown. Here we showed that knockdown of AK6 inhibits cell proliferation and promotes cell apoptosis in human testicular carcinoma (NT2 cells). Co-immunoprecipitation experiment and in vitro pull down assay identified WNK1 (with no lysine kinase-1) as one of the AK6 interacting proteins in NT2 cells. Moreover, we found that AK6 regulates the phosphorylation states of WNK1 (Thr60) and affects phosphorylation level of Akt (Ser473) upon hypotonic condition, probably affecting chloride channel and regulating ion transport and homeostasis in NT2 cells and consequently contributing to the decreased cell proliferation rate. In conclusion, AK6 regulates WNK1 phosphorylation states and affects ion homeostasis in NT2 cells. These findings provide new insights into the function of AK6 and WNK1 in human testicular carcinoma. This work also provides foundation for further mechanism study of AK6 in spermatogenesis.

hCINAP serves a critical role in hypoxia‑induced cardiomyocyte apoptosis via modulating lactate production and mitochondrial‑mediated apoptosis signaling

Acute myocardial infarction (AMI) is a major cause of heart failure and is associated with insufficient myocardial oxygen supply. However, the molecular mechanisms underlying hypoxia‑induced cardiomyocyte apoptosis are not completely understood. In the present study, the role of human coilin interacting nuclear ATPase protein (hCINAP) in cardiomyocytes was investigated. AC16 cells were divided into the following four groups: i) Small interfering (si)RNA‑control (Ctrl); (ii) siRNA‑hCINAP; (iii) empty vector; and (iv) hCINAP‑Flag. Protein expression was assessed using western blotting. MTT and apoptosis assays were conducted to detect cell viability and apoptosis, respectively. CCK8 assays and apoptosis assays were used to detect cell viability and apoptosis, respectively. hCINAP promoter activity was examined by luciferase reporter assay. hCINAP expression was induced in a hypoxia‑inducible factor‑1α‑dependent manner under hypoxic conditions. Compared with the siRNA‑Ctrl group, hCINAP knockdown inhibited apoptosis, whereas compared with the vector group, hCINAP overexpression increased apoptosis under hypoxic conditions. Mechanistically, compared with the siRNA‑Ctrl group, hCINAP knockdown decreased hypoxia‑induced lactate accumulation via regulating lactate dehydrogenase A activity. Moreover, the results indicated that hCINAP was associated with mitochondrial‑mediated apoptosis via Caspase signaling. Collectively, the present study suggested that hCINAP was an important regulator in hypoxia‑induced apoptosis and may serve as a promising therapeutic target for AMI.

hCINAP regulates the DNA-damage response and mediates the resistance of acute myelocytic leukemia cells to therapy

Acute myeloid leukemia (AML) is a genetically heterogeneous malignant disorder of the hematopoietic system, characterized by the accumulation of DNA-damaged immature myeloid precursors. Here, we find that hCINAP is involved in the repair of double-stranded DNA breaks (DSB) and that its expression correlates with AML prognosis. Following DSB, hCINAP is recruited to damage sites where it promotes SENP3-dependent deSUMOylation of NPM1. This in turn results in the dissociation of RAP80 from the damage site and CTIP-dependent DNA resection and homologous recombination. NPM1 SUMOylation is required for recruitment of DNA repair proteins at the early stage of DNA-damage response (DDR), and SUMOylated NPM1 impacts the assembly of the BRCA1 complex. Knockdown of hCINAP also sensitizes a patient-derived xenograft (PDX) mouse model to chemotherapy. In clinical AML samples, low hCINAP expression is associated with a higher overall survival rate in patients. These results provide mechanistic insight into the function of hCINAP during the DNA-damage response and its role in AML resistance to therapy.

Acute myeloid leukemia cells are often resistant to radiotherapy and chemotherapy. Here, the authors suggest that hCINAP contributes to the resistance of acute myeloid leukemia cells by regulating SUMOylation of Nucleophosmin during the DNA-damage response.

Splicing in immune cells-mechanistic insights and emerging topics

Differential splicing of mRNAs not only enables regulation of gene expression levels, but also ensures a high degree of gene-product diversity. The extent to which splicing of mRNAs is utilized as a mechanism in immune cells has become evident within the last few years. Still, only a few of these mechanisms have been well studied. In this review, we discuss some of the best-understood mechanisms, for instance the differential splicing of CD45 in T cells, as well as immunoglobulin genes in B cells. Beyond that we provide general mechanistic insights on how, when and where this process takes place and discuss the current knowledge regarding these topics in immune cells. We also highlight some of the reported links to immune-related diseases, genome-wide sequencing studies that revealed thousands of differentially spliced transcripts, as well as splicing studies on immune cells that remain mechanistically not fully understood. We thereby display potential emerging topics for future studies centered on splicing mechanisms in immune cells.

Regulatory RNPs: a novel class of ribonucleoproteins that potentially contribute to ribosome heterogeneity

Many ribonucleoproteins (RNPs), which are comprised of noncoding RNA and associated proteins, are involved in essential cellular processes such as translation and pre-mRNA splicing. One class of RNP is the small Cajal body-specific RNP (scaRNP), which contributes to the biogenesis of small nuclear RNPs (snRNPs) that are central components of the spliceosome. Three scaRNAs are internally processed, generating stable nucleolus-enriched RNAs of unknown function. Here, we provide data that show that these RNAs become part of RNPs we term regulatory RNPs (regRNPs). Most modifications within rRNA (predominantly pseudouridylation and ribose 2′-O-methylation) are conducted by small nucleolar RNPs (snoRNPs), and we provide evidence that the activity of at least some of these snoRNPs is under the control of regRNPs. Because modifications within rRNA can vary in different physiological or pathological situations, rRNA modifications are thought to be the major source of ribosome heterogeneity. Our identification of regRNPs thus provides a potential mechanism for how ribosome heterogeneity may be accomplished. This work also provides additional functional connections between the Cajal body and the nucleolus.

Summary: Processed scaRNAs give rise to a novel regulatory RNP, which regulates the modification of ribosomal RNA. These findings provide insight into the mechanisms governing ribosome heterogeneity.

Specific genomic cues regulate Cajal body assembly

The assembly of specialized sub-nuclear microenvironments known as nuclear bodies (NBs) is important for promoting efficient nuclear function. In particular, the Cajal body (CB), a prominent NB that facilitates spliceosomal snRNP biogenesis, assembles in response to genomic cues. Here, we detail the factors that regulate CB assembly and structural maintenance. These include the importance of transcription at nucleating gene loci, the grouping of these genes on human chromosomes 1, 6 and 17, as well as cell cycle and biochemical regulation of CB protein function. We also speculate on the correlation between CB formation and RNA splicing levels in neurons and cancer. The timing and location of these specific molecular events is critical to CB assembly and its contribution to genome function. However, further work is required to explore the emerging biophysical characteristics of CB assembly and the impact upon subsequent genome reorganization.

Adenylate kinase hCINAP determines self-renewal of colorectal cancer stem cells by facilitating LDHA phosphorylation

Targeting the specific metabolic phenotypes of colorectal cancer stem cells (CRCSCs) is an innovative therapeutic strategy for colorectal cancer (CRC) patients with poor prognosis and relapse. However, the context-dependent metabolic traits of CRCSCs remain poorly elucidated. Here we report that adenylate kinase hCINAP is overexpressed in CRC tissues. Depletion of hCINAP inhibits invasion, self-renewal, tumorigenesis and chemoresistance of CRCSCs with a loss of mesenchymal signature. Mechanistically, hCINAP binds to the C-terminal domain of LDHA, the key regulator of glycolysis, and depends on its adenylate kinase activity to promote LDHA phosphorylation at tyrosine 10, resulting in the hyperactive Warburg effect and the lower cellular ROS level and conferring metabolic advantage to CRCSC invasion. Moreover, hCINAP expression is positively correlated with the level of Y10-phosphorylated LDHA in CRC patients. This study identifies hCINAP as a potent modulator of metabolic reprogramming in CRCSCs and a promising drug target for CRC invasion and metastasis.

Coordinating cell cycle-regulated histone gene expression through assembly and function of the Histone Locus Body

Metazoan replication-dependent (RD) histone genes encode the only known cellular mRNAs that are not polyadenylated. These mRNAs end instead in a conserved stem-loop, which is formed by an endonucleolytic cleavage of the pre-mRNA. The genes for all 5 histone proteins are clustered in all metazoans and coordinately regulated with high levels of expression during S phase. Production of histone mRNAs occurs in a nuclear body called the Histone Locus Body (HLB), a subdomain of the nucleus defined by a concentration of factors necessary for histone gene transcription and pre-mRNA processing. These factors include the scaffolding protein NPAT, essential for histone gene transcription, and FLASH and U7 snRNP, both essential for histone pre-mRNA processing. Histone gene expression is activated by Cyclin E/Cdk2-mediated phosphorylation of NPAT at the G1-S transition. The concentration of factors within the HLB couples transcription with pre-mRNA processing, enhancing the efficiency of histone mRNA biosynthesis.

The ATPase hCINAP regulates 18S rRNA processing and is essential for embryogenesis and tumour growth

Dysfunctions in ribosome biogenesis cause developmental defects and increased cancer susceptibility; however, the connection between ribosome assembly and tumorigenesis remains unestablished. Here we show that hCINAP (also named AK6) is required for human 18S rRNA processing and 40S subunit assembly. Homozygous CINAP^−/− mice show embryonic lethality. The heterozygotes are viable and show defects in 18S rRNA processing, whereas no delayed cell growth is observed. However, during rapid growth, CINAP haploinsufficiency impairs protein synthesis. Consistently, hCINAP depletion in fast-growing cancer cells inhibits ribosome assembly and abolishes tumorigenesis. These data demonstrate that hCINAP reduction is a specific rate-limiting controller during rapid growth. Notably, hCINAP is highly expressed in cancers and correlated with a worse prognosis. Genome-wide polysome profiling shows that hCINAP selectively modulates cancer-associated translatome to promote malignancy. Our results connect the role of hCINAP in ribosome assembly with tumorigenesis. Modulation of hCINAP expression may be a promising target for cancer therapy.

Perturbations in ribosome biogenesis affect development and increase cancer susceptibility. Here, the authors show that hCINAP is required for 18S rRNA processing, is highly expressed in cancers, and promotes cancer cell growth by upregulating the translation of cancer-associated genes.

hCINAP negatively regulates NF-κB signaling by recruiting the phosphatase PP1 to deactivate IKK complex

Tight regulation of nuclear factor-κB (NF-κB) signaling is essential to maintain homeostasis in immune system in response to various stimuli, which has been studied extensively and deeply. However, the molecular mechanisms responsible for its negative regulation are not completely understood. Here we demonstrate that human coilin-interacting nuclear ATPase protein (hCINAP) is a novel negative regulator in NF-κB signaling by deactivating IκB kinase (IKK) complex. In response to TNF stimulation, hCINAP dynamically associates with IKKα and IKKβ and inhibits IKK phosphorylation. Notably, hCINAP directly interacts with the catalytic subunits of protein phosphatase 1 (PP1) and mediates the formation of IKK-hCINAP-PP1 complex, serving as an adaptor protein that recruits PP1 to dephosphorylate IKK. Furthermore, decreased levels of hCINAP are observed in several inflammatory diseases with NF-κB hyperactivity. Our study suggests a novel mechanism underlying deactivation of IKK and provides new insight into the negative regulation of NF-κB signaling.

RNA mimicry by the fap7 adenylate kinase in ribosome biogenesis

The structure of a ribosome assembly factor in complex bound to a ribosomal protein uncovers a chaperoning function that uses RNA mimicry and is regulated by ATP hydrolysis.

During biogenesis of the 40S and 60S ribosomal subunits, the pre-40S particles are exported to the cytoplasm prior to final cleavage of the 20S pre-rRNA to mature 18S rRNA. Amongst the factors involved in this maturation step, Fap7 is unusual, as it both interacts with ribosomal protein Rps14 and harbors adenylate kinase activity, a function not usually associated with ribonucleoprotein assembly. Human hFap7 also regulates Cajal body assembly and cell cycle progression via the p53–MDM2 pathway. This work presents the functional and structural characterization of the Fap7–Rps14 complex. We report that Fap7 association blocks the RNA binding surface of Rps14 and, conversely, Rps14 binding inhibits adenylate kinase activity of Fap7. In addition, the affinity of Fap7 for Rps14 is higher with bound ADP, whereas ATP hydrolysis dissociates the complex. These results suggest that Fap7 chaperones Rps14 assembly into pre-40S particles via RNA mimicry in an ATP-dependent manner. Incorporation of Rps14 by Fap7 leads to a structural rearrangement of the platform domain necessary for the pre-rRNA to acquire a cleavage competent conformation.

Ribosomes are the cellular machines responsible for all protein synthesis. In eukaryotes, the assembly of ribosomes from their protein and RNA components is extremely complicated and involves more than 200 nonribosomal factors—three times the number of proteins in the mature complex. Among these factors, the Fap7 protein is particularly intriguing because it interacts with the small subunit ribosomal protein Rps14 and it exhibits adenylate kinase activity—a molecular function more commonly associated with regulating ATP/ADP levels than assembling protein–RNA complexes. Combining structural and biochemical analysis of the Rps14–Fap7 complex, we show that Fap7 uses protein side chains to mimic RNA contacts, rendering the interaction of Rps14 with ribosomal RNA or with Fap7 competitive and mutually exclusive. Once bound, Rps14 blocks the substrate-binding cavity of Fap7, and ATP hydrolysis will then break the Fap7–Rps14 complex apart. At the same time, the ribosome structure at the location where Rps14 binds is disrupted when the Fap7/Rps14 complex is formed, and this process is regulated by ATP binding and hydrolysis. Our model is thus that Fap7 temporarily removes Rps14 from the ribosome to enable a conformational change of the ribosomal RNA that is needed for the final maturation step of the small ribosomal subunit.

Fam118B, a newly identified component of Cajal bodies, is required for Cajal body formation, snRNP biogenesis and cell viability

Cajal bodies are specialized and dynamic compartments in the nucleus that are involved in the biogenesis of small nuclear ribonucleoproteins (snRNPs). Because of the dynamic and varied roles of Cajal bodies, it is of great interest to identify the components of Cajal bodies to better understand their functions. We performed a genome-wide screen to identify proteins that colocalize with coilin, the marker protein of Cajal bodies. In this study, we identified and characterized Fam118B as a newly discovered component of Cajal bodies. Fam118B is widely expressed in a variety of cell lines derived from various origins. Overexpression of Fam118B changes the canonical morphology of Cajal bodies, whereas depletion of Fam118B disrupts the localization of components of Cajal bodies, including coilin, the survival of motor neuron protein (SMN) and the Sm protein D1 (SmD1, also known as SNRPD1). Moreover, depletion of Fam118B reduces splicing capacity and inhibits cell proliferation. In addition, Fam118B associates with coilin and SMN proteins. Fam118B depletion reduces symmetric dimethylarginine modification of SmD1, which in turn diminishes the binding of SMN to this Sm protein. Taken together, these data indicate that Fam118B, by regulating SmD1 symmetric dimethylarginine modification, plays an important role in Cajal body formation, snRNP biogenesis and cell viability.

Molecular and functional characterization of a Trypanosoma cruzi nuclear adenylate kinase isoform

Trypanosoma cruzi, the etiological agent of Chagas' disease, is an early divergent eukaryote in which control of gene expression relies mainly in post-transcriptional mechanisms. Transcription levels are globally up and down regulated during the transition between proliferating and non-proliferating life-cycle stages. In this work we characterized a nuclear adenylate kinase isoform (TcADKn) that is involved in ribosome biogenesis. Nuclear adenylate kinases have been recently described in a few organisms, being all related to RNA metabolism. Depending on active transcription and translation, TcADKn localizes in the nucleolus or the cytoplasm. A non-canonical nuclear localization signal was mapped towards the N-terminal of the protein, being the phosphate-binding loop essential for its localization. In addition, TcADKn nuclear exportation depends on the nuclear exportation adapter CRM1. TcADKn nuclear shuttling is governed by nutrient availability, oxidative stress and by the equivalent in T. cruzi of the mammalian TOR (Target of Rapamycin) pathway. One of the biological functions of TcADKn is ribosomal 18S RNA processing by direct interaction with ribosomal protein TcRps14. Finally, TcADKn expression is regulated by its 3′ UTR mRNA. Depending on extracellular conditions, cells modulate protein translation rates regulating ribosome biogenesis and nuclear adenylate kinases are probably key components in these processes.

Infection with Trypanosoma cruzi produces a condition known as Chagas disease which affects at least 17 million people. Adenylate kinases, so called myokinases, are involved in a wide variety of processes, mainly related to their role in nucleotide interconversion and energy management. Recently, nuclear isoforms have been described in several organisms. This “atypical” isoform in terms of primary structure was associated to ribosomes biogenesis in yeast and to Cajal body organization in humans. Moreover nuclear adenylate kinases are essential for maintaining cellular homeostasis. In this manuscript we characterized T. cruzi nuclear adenylate kinase (TcADKn). TcADKn localizes in the nucleolus or cell cytoplasm. Nuclear shuttling mechanisms were also studied for the first time, being dependent on nutrient availability, oxidative stress and by the equivalent of the mammalian TOR pathway in T. cruzi. Furthermore we characterized the signals involved in nuclear importation and exportation processes. In addition, TcADKn expression levels are regulated at an mRNA level, being its 3′UTR involved in this process. These findings are the first steps in the understanding of ribosome processing in trypanosomatids.

hCINAP is a novel regulator of ribosomal protein-HDM2-p53 pathway by controlling NEDDylation of ribosomal protein S14

The tumor-suppressor p53 provides a critical brake on tumor development. HDM2 (human double-minute 2), a p53 E3 ubiquitin ligase, is the principal cellular antagonist of p53. Mounting evidence has suggested that ribosomal proteins (RPs) modulate HDM2-p53 as a novel pathway for regulating p53 signaling. However, the upstream regulators that mediate RP-HDM2-p53 circuits remain poorly understood. Here we identify human coilin-interacting nuclear ATPase protein (hCINAP) as an interacting partner of ribosomal protein S14 (RPS14). RPS14 stabilized and activated p53 by inhibiting HDM2-mediated p53 polyubiquitination and degradation. More importantly, RPS14 was specifically modified with NEDD8 and hCINAP inhibited RPS14 NEDDylation by recruiting NEDD8-specific protease 1. The decrease in RPS14 NEDDylation led to reduced stability and incorrect localization of RPS14, thereby attenuating the interaction between RPS14 and HDM2. Free HDM2 stimulated p53 polyubiquitination and degradation. In conclusion, we demonstrate that hCINAP acts as a novel regulator of RPS14-HDM2-p53 by regulating the interaction between RPS14 and HDM2 through the control of RPS14 NEDDylation. These findings suggest that hCINAP is an important regulator of RP-HDM2-p53 pathway and a potential anticancer drug target.

Cajal bodies: where form meets function

The cell nucleus contains dozens of subcompartments that separate biochemical processes into confined spaces. Cajal bodies (CBs) were discovered more than 100 years ago, but only extensive research in the past decades revealed the surprising complexity of molecular and cellular functions taking place in these structures. Many protein and RNA species are modified and assembled within CBs, which have emerged as a meeting place and factory for ribonucleoprotein (RNP) particles involved in splicing, ribosome biogenesis and telomere maintenance. Recently, a distinct structure near histone gene clusters--the Histone locus body (HLB)--was discovered. Involved in histone mRNA 3'-end formation, HLBs can share several components with CBs. Whether the appearance of distinct HLBs is simply a matter of altered affinity between these structures or of an alternate mode of CB assembly is unknown. However, both structures share basic assembly properties, in which transcription plays a decisive role in initiation. After this seeding event, additional components associate in random order. This appears to be a widespread mechanism for body assembly. CB assembly encompasses an additional layer of complexity, whereby a set of pre-existing substructures can be integrated into mature CBs. We propose this as a multi-seeding model of CB assembly.

ATP binding to hemoglobin response gene 1 protein is necessary for regulation of the mating type locus in Candida albicans

HBR1 (hemoglobin response gene 1) is an essential gene in Candida albicans that positively regulates mating type locus MTLα gene expression and thereby regulates cell type-specific developmental genes. Hbr1p contains a phosphate-binding loop (P-loop), a highly conserved motif characteristic of ATP- and GTP-binding proteins. Recombinant Hbr1p was isolated in an oligomeric state that specifically bound ATP with K(d) ∼2 μM. ATP but not ADP, AMP, GTP, or dATP specifically protected Hbr1p from proteolysis by trypsin. Site-directed mutagenesis of the highly conserved P-loop lysine (K22Q) and the less conserved glycine (G19S) decreased the binding affinity for soluble ATP and ATP immobilized through its γ-phosphate. ATP bound somewhat more avidly than ATPγS to wild type and mutant Hbr1p. Although Hbr1p exhibits sequence motifs characteristic of adenylate kinases, and adenylate kinase and ATPase activities have been reported for the apparent human ortholog of Hbr1p, assays for adenylate kinase activity, autophosphorylation, and ATPase activity proved negative. Overexpression of wild type but not the mutant forms of Hbr1p restored MTlα2 expression in an HBR1/hbr1 mutant, indicating that ATP binding to the P-loop is necessary for this function of Hbr1p.

The nuclear ATPase/adenylate kinase hCINAP is recruited to perinucleolar caps generated upon RNA pol.II inhibition

hCINAP is an atypical nucleoplasmic enzyme, combining structural features of adenylate kinases and ATPases, which exhibits dual enzymatic activity. It interacts with the Cajal Body marker coilin and its level of expression and enzymatic activity influence Cajal Body numbers. Here we show that upon specific transcriptional inhibition of RNA pol.II, hCINAP segregates in perinuclear caps identified as Dark Nucleolar Caps (DNCs). These are distinct from perinucleolar caps where coilin and fibrillarin (both Cajal Body components) accumulate. In DNCs, hCINAP co-localizes with Paraspeckle Protein (PSP1) and also co-segregates with PSP1, and not coilin, in nuclear and nucleolar foci upon UV irradiation.