The involvement of caspases in the CD95(Fas/Apo-1)- but not swelling-induced cellular taurine release from Jurkat T-lymphocytes

Following a delay of 45 min, stimulation of the CD95 (Fas/Apo-1)-receptor in Jurkat T-lymphocytes leads to the release of the osmolyte taurine, an event coinciding with apoptotic cell shrinkage. The present study has been performed to elucidate the cellular mechanisms involved in CD95-induced taurine release as compared to swelling-induced taurine release, and to explore whether taurine modifies apoptotic DNA fragmentation and cell shrinkage. Taurine release stimulated by osmotic cell swelling is insensitive to the tyrosine kinase inhibitor herbimycin A and the caspase inhibitor z-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD) but is blunted in the absence of extracellular Ca2+. Conversely, the Ca2+ ionophore ionomycin stimulates taurine release. However, the taurine release following CD95 stimulation is not paralleled by an increase of cytosolic Ca2+ and not inhibited by complexation of extracellular Ca2+. None of herbimycin A, the phosphatase inhibitor vanadate, spingomyelinase or Lck56 deficiency prevent CD95-induced taurine release. In contrast, the caspase inhibitor zVAD, but not the caspase inhibitor Ac-Tyr-Val-Ala-Asp-chloromethylketone (YVAD), almost abolishes CD95-induced taurine release. Both caspase inhibitors blunt CD95-induced cell shrinkage and DNA fragmentation, zVAD being more effective than YVAD. Preloading of the cells with 40 mM taurine but not with 40 mM mannitol significantly inhibits CD95-induced DNA fragmentation (by 28%) and apoptotic cell shrinkage (by 25%). In conclusion, CD95-receptor triggering leads to caspase-dependent stimulation of cellular taurine release, which facilitates, but is not sufficient for, the triggering of apoptotic DNA fragmentation and cell shrinkage.     

Inhibition of Jurkat-T-lymphocyte Na+/H+-exchanger by CD95(Fas/Apo-1)-receptor stimulation

Mitogenic factors are known to stimulate the Na+/H+-exchanger (NHE), leading to cytosolic alkalinization and/or cell swelling. Conversely, a hallmark of apoptosis is cell shrinkage and CD95-induced apoptosis has been reported to be paralleled by cytosolic acidification. To assess whether the CD95-receptor regulates NHE activity in Jurkat T-lymphocytes, we performed conventional BCECF fluorescence measurements and SNARF flow cytometric analysis (FACS). The recoveries from acidifications following application of butyrate or a NH3 pulse were both abolished by a specific NHE-inhibitor, HOE694, indicating that they fully depend on NHE activity. Thus they were taken as a measure of NHE activity. CD95-receptor stimulation caused a cytosolic acidification and blunted the recovery from acidification following application of butyrate or a NH3 pulse. Moreover, the NHE-dependent alkalinization following osmotic cell shrinkage was almost abolished by CD95-receptor stimulation. As apparent from the effect of osmotic cell shrinkage, inhibition of the NHE by CD95-receptor stimulation was absent in Lck56-deficient J-CaM1.6 cells and restored by retransfection of J-CaM1.6-cells with Lck56. CD95-receptor stimulation led within 4 h to a decrease of cellular ATP which could contribute to NHE inhibition. Treatment of Jurkat cells with the NHE inhibitor HOE694 accelerated CD95-induced DNA fragmentation. In conclusion, CD95-receptor stimulation inhibits NHE activity through a mechanism that depends directly or indirectly on the activation of the Src-like kinase Lck56. This effect contributes to CD95-induced cytosolic acidification, DNA fragmentation and cell shrinkage.     

Role of dietary starches in induction of immune cells apoptosis in Wistar rats

We compared the effects of maize and potato starch on Wistar rats. The level of oligonucleosomal DNA fragmentation increased in thymic and splenic cells of animals receiving potato starch. The expression of Fas/CD95/Apo-1 apoptosis receptor in lymphocytes and their response to exogenous ceramide was also higher in rats receiving potato starch compared to the reference group. This led to intensification of spontaneous and induced apoptosis and decreased cell content in immune organs. The authors conclude that higher resistance of potato starch to digestive enzymes leads to modification of cell metabolism and induction of ceramide-dependent apoptosis in immune cells.     

CEACAM1 (CD66a) mediates delay of spontaneous and Fas ligand-induced apoptosis in granulocytes

Granulocytes form the first and fastest line of defense against pathogenic infections. Their survival is limited by apoptosis, a process that is critical for the resolution of inflammation. Pro-apoptotic and pro-inflammatory cytokines, as well as several receptors, can alter the lifespan of granulocytes. Here we report that the carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1, CD66a) is involved in the regulation of granulocyte survival. Until now CEACAM1 is described to control cell proliferation, cell migration, tumor growth, angiogenesis and diverse leukocyte functions. However, very little is known about its role in granulocytes. We found that CEACAM1 expression in resting rat granulocytes is significantly higher than in other leukocyte subtypes. Stimulation led to a strongly increased CEACAM1 cell surface expression and to release of soluble CEACAM1. DNA fragmentation assays and annexin V staining revealed that binding of CEACAM1-specific antibodies, Fab fragments and soluble CEACAM1-Fc constructs to cell surface-expressed CEACAM1 causes a delay of spontaneous and Fas ligand (CD95L)-induced apoptosis. Tyrosine phosphorylation of CEACAM1-L, its association with SHP-1, the activation of Erk1/2 and caspase-3 appeared to be crucial for the CEACAM1-mediated anti-apoptotic effect. These findings provide evidence that CEACAM1 influences the resolution of inflammation by prolonging the survival of rat granulocytes.     

Hyperosmotic activation of the CD95 death receptor system

Apoptosis is characterized by cell shrinkage, nuclear condensation, DNA fragmentation and apoptotic body formation. These features distinguish apoptosis from other types of cell death, such as necrosis. Whereas some signs of apoptosis, such as externalization of phosphatidylserine, altered mitochondrial function or activation of caspases are cell type- and death signal-dependent, apoptotic cell volume decrease (AVD) is an early and ubiquitous event and little is known about the signalling events, which are localized upstream of the plasma membrane transport steps leading to AVD and the proapoptotic events, which are induced by osmolyte loss and cell shrinkage. In hepatocytes hyperosmotic shrinkage sensitizes the cells towards CD95 ligand-induced apoptosis by activating the CD95 system. This complex process with a NADPH oxidase-derived reactive oxygen species signal as an important upstream event, allows via Yes, JNK and epidermal growth factor-receptor activation for CD95 tyrosine phosphorylation as a prerequisite for CD95 targeting to the plasma membrane and formation of the death inducing signalling complex. Other covalent modifications such as CD95-tyrosine-nitration or CD95-serine/threonine-phosphorylation can interfere with the CD95 activation process. The findings not only provide a mechanistic explanation for the high susceptibility of dehydrated cells for apoptosis, but also give insight into the role of AVD.     

The CD40-induced protection against CD95-mediated apoptosis is associated with a rapid upregulation of anti-apoptotic c-FLIP

CD95/Fas and CD40 receptors are important regulators of cell survival during germinal center reaction. In this study we used a human follicular lymphoma cell line, HF1A3, to study molecular mechanisms of CD95-mediated apoptosis and CD40-induced rescue from apoptosis. CD95 stimulation induced activation of caspase-8 and -3, collapse of mitochondrial membrane potential (DeltaPsim), release of cytochrome c and fragmentation of nuclear DNA. All these apoptotic events were abrogated, when cells were pretreated with CD40 antibodies before CD95 stimulation. CD40 induced a rapid up regulation of both short and long isoforms of c-FLIP, as these proteins were detectable 4h after receptor stimulation. The induction of c-FLIP as well as the anti-apoptotic function of CD40 was completely abolished when NF-kappaB activity was inhibited by a selective inhibitor PDTC. We conclude that the anti-apoptotic signaling of CD40 involves NF-kappaB-mediated induction of c-FLIP proteins which can interfere with caspase-8 activation. However, it remains to be seen whether c-FLIP proteins are the only one ones involved in CD40-mediated protection.     

CD40.FasL inhibits human T cells: evidence for an auto-inhibitory loop-back mechanism

A chimeric CD40.FasL (CD40-CD95L) protein was designed with the combined capacities to bind to two surface receptors on activated T cells, CD40 ligand (CD40L; CD154) and Fas receptor (CD95). CD40.FasL, once tethered to the cell surface via one of its ends, can transmit a signal via its other end. In principle, simultaneous triggering from both ends is possible, and thus there is the intriguing potential for 'auto-inhibition' if such dual triggering occurs on the same cell itself. Several lines of evidence support this mechanism: (i) CD40.FasL is cytotoxic to Fas receptor-positive cell lines of different cell lineages, (ii) CD40.FasL's function is potentiated when there is enforced expression of CD40L on target cells, (iii) CD40.FasL inhibition does not require intercellular contact, as demonstrated by soft agar clone formation and cell dilution analysis and (iv) introduction of exogenous CD40 into the system interferes with CD40.FasL inhibition. Taken together, these data are consistent with a 'loop-back' inhibitory mechanism within individual activated (CD40L and Fas receptor expressing) T cells causing suicide of these T cells. Significantly, this type of fusion protein provides a unique way to confine immunoinhibition to activated T cells.     

Feedback Regulation of Mitochondria by Caspase-9 in the B Cell Receptor-Mediated Apoptosis

During the germinal centre reaction (GC), B cells with non-functional or self-reactive antigen receptors are negatively selected by apoptosis to generate B cell repertoire with appropriate antigen specificities. We studied the molecular mechanism of Fas/CD95- and B cell receptor (BCR)-induced apoptosis to shed light on the signalling events involved in the negative selection of GC B cells. As an experimental model, we used human follicular lymphoma (FL) cell line HF1A3, which originates from a GC B cell, and transfected HF1A3 cell lines overexpressing Bcl-x_L, c-FLIP_long or dominant negative (DN) caspase-9. Fas-induced apoptosis was dependent on the caspase-8 activation, since the overexpression of c-FLIP_long, a natural inhibitor of caspase-8 activation, blocked apoptosis induced by Fas. In contrast, caspase-9 activation was not involved in Fas-induced apoptosis. BCR-induced apoptosis showed the typical characteristics of mitochondria-dependent (intrinsic) apoptosis. Firstly, the activation of caspase-9 was involved in BCR-induced DNA fragmentation, while caspase-8 showed only marginal role. Secondly, overexpression of Bcl-x_L could block all apoptotic changes induced by BCR. As a novel finding, we demonstrate that caspase-9 can enhance the cytochrome-c release and collapse of mitochondrial membrane potential (Δ Ψ _m) during BCR-induced apoptosis. The requirement of different signalling pathways in apoptosis induced by BCR and Fas may be relevant, since Fas- and BCR-induced apoptosis can thus be regulated independently, and targeted to different subsets of GC B cells.     

Spontaneous and Fas-Induced Apoptotic Cell Death in Aged Neutrophils

On the basis of the strict analogies between polymorphonuclear cell (PMN) alterations in the aging and depressed functional capacities displayed by apoptotic PMN, we investigated the possible occurrence of age-associated changes in neutrophil apoptosis, either spontaneous or induced by Fas antigen (CD95) activation. In both cases, old subjects exhibited a time course kinetics of neutrophil apoptosis, as assessed by morphologic and quantitative DNA fragmentation analysis, which overlapped that observed in the young. These findings were confirmed by DNA ladder analysis, showing a progressive increase in DNA cleavage products in cells cultured in medium alone or added with agonistic anti-Fas IgM (CH-11) monoclonal antibodies (mAbs), after 12 and 6 hr of incubation, respectively. Aged purified neutrophils constitutively expressed CD95, at levels similar to those observed in the young. Moreover, although we failed to detect Fas ligand expression on PMN surface, treatment of cell cultures with antagonistic anti-Fas IgG1 (ZB4) mAbs determined a significant inhibition of spontaneous apoptosis in neutrophils from both groups of subjects, thus suggesting that the Fas/Fas ligand system is in fact involved in such an event. The results indicate that the overall intrinsic mechanisms regulating neutrophil cell death are not affected by age. Yet aged neutrophils showed a diminished capacity to be rescued by proinflammatory mediators, such as granulocyte–monocyte colony-stimulating factor, granulocyte colony-stimulating factor, and bacterial lipopolysaccharide, following Fas activation. This may hamper the accumulation of functionally active cells in inflammatory areas in vivo, thus contributing to the increased susceptibility of elderly individuals to life-threatening infections.     

Inhibition of CD95 (Fas/Apo1)-mediated apoptosis by vaccinia virus WR

Stimulation of the CD95 (Apo-1/Fas) molecule either by the CD95 ligand or by monoclonal antibodies induces programmed cell death by apoptosis in a variety of cell lines and primary cells. In this study we observed that infection of B lymphoblast and T lymphoblast cell lines with vaccinia virus strain WR and recombinant vaccinia WR constructs, but not strain Copenhagen, rendered cells refractory to CD95-mediated apoptosis. In particular, vaccinia virus infection suppressed anti-CD95 antibody-induced membrane disintegration, apoptotic nuclear morphology of cells, and DNA fragmentation. Inhibition of apoptosis was not mediated by CD95 down-regulation or reduced binding of anti-CD95 antibody to infected cells, and occurred at a time point when cellular metabolism was not yet affected by the lytic vaccinia virus infection. Vaccinia virus (WR)-infected cells were resistant to CD95 ligand–CD95-mediated lysis by CD4⁺ and CD8⁺, T lymphocytes. Because cytolysis mediated by CD95 is one of two major mechanisms used by cytotoxic T lymphocytes to kill target cells, inhibition of CD95-mediated apoptosis may constitute a novel immune escape mechanism for this virus. Additionally, this mechanism may contribute to the higher pathogenicity of vaccinia virus strain WR compared with strain Copenhagen.     

Apoptosis induction by interleukin-2-activated cytotoxic lymphocytes in a squamous cell carcinoma cell line and Daudi cells - involvement of reactive oxygen species-dependent cytochrome c and reactive oxygen species-independent apoptosis-inducing factors

Investigation of the induction of apoptosis by cytotoxic lymphocytes has mainly focused on the signalling associated with Fas and its adaptor proteins. The signal pathway via mitochondria, however, has not been sufficiently elucidated in cytotoxic lymphocyte-induced apoptosis. We examined the release of mitochondrial proapoptotic factors by lymphokine-activated killer (LAK) cells in two cell lines. LAK cell-induced DNA fragmentation of the target cells was suppressed to approximately 50% of control levels by the addition of neutralizing monoclonal antibody to Fas and a granzyme B inhibitor. When intracellular reactive oxygen species (ROS) were scavenged, the LAK cell-induced DNA fragmentation was decreased to approximately 60% of the non-treated cell level. Co-cultivation of Daudi cells with LAK cells increased cytosolic and mitochondrial ROS levels. Activation of procaspase-3 and apoptosis by treatment of oral squamous cell carcinoma cells (OSC) with LAK cells was partially inhibited by pretreatment of OSC cells with ROS scavengers and mitochondrial complex inhibitors. Furthermore, cytochrome c and apoptosis-inducing factor (AIF) were released from mitochondria by OSC cell treatment with supernatants of LAK cells. The supernatant-induced cytochrome c release was suppressed by mitochondrial complex inhibitors, but the inhibitors did not inhibit the release of AIF. These results indicate that LAK cells induce target cell apoptosis via not only the Fas/Fas ligand system and granzyme B, but also ROS-dependent cytochrome c and ROS-independent AIF release.     

Activation-induced NK cell death triggered by CD2 stimulation

Both T cells and natural killer (NK) cells express CD2, the target of an alternative activation pathway that induces the proliferation of both cell types. The mitogenic response to CD2 ligation requires the co-expression of CD3 : TCR in T cells and FcγRIII in NK cells, suggesting that these receptors are involved in transducing the response initiated by CD2. The ability of FcγRIII to trigger the activation-induced death of IL-2-primed NK cells led us to investigate the potential for CD2 to trigger activation-induced NK cell death. Our results reveal that the same anti-CD2 monoclonal antibodies (mAb) that activate freshly isolated NK cells induce apoptosis in IL-2-primed NK cells. CD2-induced apoptosis results in chromatin condensation, DNA fragmentation and cleavage of caspase-3. Activation-induced NK cell death triggered by CD2 ligation is extremely rapid (DNA fragmentation is first observed at 90 min) and it is not inhibited by neutralizing antibodies reactive with TNF-α or Fas ligand. Whereas mAb reactive with distinct CD2 epitopes (i.e. T11.1, T11.2, and T11.3) are required for activation-induced T cell death, mAb reactive with a single CD2 epitope are sufficient for activation-induced NK cell death. The ability of CD2, CD16, and CD94 to induce apoptosis in IL-2-primed lymphocytes suggests that cytokine priming changes the response to a signaling cascade that is common to each of these activation receptors.     

Hypotonicity stimulates translocation of ICln in neonatal rat cardiac myocytes

 Cell volume expansion stimulates the efflux of solutes, including the amino acid taurine, to accomplish a regulatory volume decrease (RVD). One protein that may play a role in taurine efflux is the cytosolic protein ICln. In rat neonatal cardiac myocytes under isotonic conditions, ICln is found predominantly (greater than 90%) in the cytosol. However, after cell volume expansion by exposure to hypotonic medium, ICln rapidly translocates to the particulate fraction (the Triton X-114-insoluble fraction). After 2 min in hypotonic medium the percentage of ICln in the particulate fraction increases to 30%, 46% at 5 min, 40% at 10 min, and 25% at 30 min. The time course of this response is similar to that of hypotonicity-stimulated taurine efflux. Hypotonicity-stimulated taurine efflux as well as ICln translocation parallel the reduction in medium osmolarity. As osmolarity decreases, taurine efflux and ICln movement increase. The movement of ICln from the particulate back to the cytosolic fraction is accelerated when volume-expanded cells are returned to isotonic medium. When ICln is analyzed under non-denaturing conditions, a dimer is detected in the particulate fraction of volume-expanded cells, along with the monomer. This dimer is not detected in the cytosol. Treatment of the particulate fraction from volume-expanded cells with the lyotropic agent KSCN caused release of ICln but not Na-K-ATPase into the soluble fraction, indicating that translocated ICln associates with membranes in the particulate fraction rather than inserting into them. Received: 31 October 1997 / Received after revision and accepted: 23 March 1998     

Human papillomavirus type 16 E5 protein localizes to the Golgi apparatus but does not grossly affect cellular glycosylation

Summary.  The E5 protein of papillomaviruses is a strongly hydrophobic membrane protein that can associate with the 16 kDa protein subunit of the vacuolar proton ATPase in endosomes and the Golgi apparatus resulting in raise of intraorganelle pH. We demonstrate that E5 of human papillomavirus type 16 (HPV16) when transfected into human keratinocytes localizes to the Golgi. Using FACS analysis and western blotting with a variety of lectins as well as analysing the sialylation status of a specific cell surface glycoprotein CD95 (APO-1/Fas), we show that HPV16 E5 does not grossly affect cellular glycosylation, a main Golgi function. Received March 6, 2000 Accepted May 18, 2000     

Hypericin induced death receptor-mediated apoptosis in photoactivated tumor cells

Nasopharyngeal carcinoma (NPC) is a malignant disease of the head/neck region with a 5-year survival level of approximately 65%. To explore the novel therapeutic strategies in the management of this disease, the potential effects of photodynamic therapy (PDT) in NPC cells were investigated. PDT, a new mode of treatment, is based on the combined use of light-absorbing compounds and light irradiation. Two human NPC cells such as, poorly differentiated (NPC/CNE2) and moderately differentiated (NPC/TW0-1) and other types of tumor cells like colon (CCL-220.1) and bladder (SD) undergo rapid apoptosis when treated with PDT sensitized with hypericin (HY). It has been shown that this compound has a strong photodynamic effect on tumors and viruses. However, the initiating events of PDT sensitized HY-induced apoptosis are not identified completely. In this study, we sought to determine whether Fas/FasL upregulation and involvement of mitochondrial events are an early event in HY-treated PDT induced apoptosis. Loss of mitochondrial transmembrane potential, release of cytochrome c, involvement of caspases 8 and 3 and the status caspase-3 specific substrate PARP, were evaluated in PDT treated tumor cells. Photosensitization of HY enhanced both CD95/CD95L expression and induced CD95-signaling dependent cell death in all tumor cell lines studied. CD95/CD95L expression appeared within 2 h following light irradiation and appeared to be a principal event in PDT induced apoptosis. Furthermore, these results indicate that release of mitochondrial cytochrome c into the cytoplasm within 2-3 h post PDT is a secondary event following the activation of initiator caspase-8 preceding Apaf-1, caspase-9 and caspase-3 activation, cleavage of PARP and DNA fragmentation.     

Fas ligand-mediated depletion of CD4 and CD8 lymphocytesby monomeric HIV-1-gp120

Summary.  In order to determine in what condition and by what mechanism gp 120 can deplete not only CD4 but also CD8 T cells, an in vitro system was established in which peripheral blood lymphocytes from healthy donors were treated with recombinant gp 120. We found that gp 120 can deplete both CD4 and CD8 T cells when they have recently been activated and are exposed to IL-2-deficient conditions. Bioassay of the Fas ligand (FasL) demonstrated augmented expression and release of soluble FasL by CD4 T cells in the supernatant of this culture. The administration of anti-FasL mAb and anti-Fas mAb, both of which exhibit neutralizing activity, completely abolished the depletion of these two T cell populations in culture. Based on these findings, we concluded that FasL depletes Fas antigen expressing CD4 and CD8 T cells by programmed cell death. Received December 18, 1996 Accepted May 2, 1997     

Kinetics and signaling requirements of CD40-mediated protection from B cell receptor-induced apoptosis

In the present study we used a human follicular lymphoma cell line, HF1A3, as an in vitro model for the antigen-driven selection process in germinal centers. Apoptosis can be induced in HF1A3 cells by B cell receptor (BCR) stimulation, but the molecular mechanisms and kinetics of this process are largely unknown. We demonstrate here that there is over 12 h delay between receptor activation and the execution phase of apoptosis, i.e. disruption of mitochondrial membrane potential, release of cytochrome c from mitochondria, caspase-3 activation and DNA fragmentation. New protein synthesis is required for mitochondrial alterations and subsequent apoptosis to occur, as these processes are completely blocked by the protein synthesis inhibitor cycloheximide. All the apoptotic events induced by BCR triggering are completely reversed by CD40 ligation with anti-CD40 antibody. CD40 ligation can reverse the apoptotic process in HF1A3 cells almost until the first mitochondrial events take place demonstrating that CD40-mediated protection operates very fast and at or before mitochondrial phase of apoptosis. Using specific inhibitors of cell signaling we could demonstrate that Raf-extracellular signal-regulated kinase, phosphatidylinositol 3-kinase, p38 or protein kinase C activation pathways are not involved in CD40-mediated protection from BCR-induced apoptosis in HF1A3 cells.     

Fas/CD95-mediated apoptosis of type II cells is blocked by Toxoplasma gondii primarily via interference with the mitochondrial amplification loop

The intracellular protozoan Toxoplasma gondii induces persistent infections in various hosts and is an important opportunistic pathogen of humans with immature or deficient immune responses. The ability to survive intracellularly largely depends on the blocking of different proapoptotic signaling cascades of its host cell. Fas/CD95 triggers an apoptotic cascade that is crucial for immunity and the outcome of infectious diseases. We have determined the mechanism by which T. gondii counteracts death receptor-mediated cell death in type II cells that transduce Fas/CD95 ligation via caspase 8-mediated activation of the mitochondrial amplification loop. The results showed that infection with T. gondii significantly reduced Fas/CD95-triggered apoptosis in HeLa cells by inhibiting the activities of initiator caspases 8 and 9 and effector caspase 3/7. Parasitic infection dose dependently diminished cleavage of caspase 8, the BH3-only protein Bid, and the downstream caspases 9 and 3. Importantly, interference with Fas/CD95-triggered caspase 8 and caspase 3/7 activities after parasitic infection was largely dependent on the presence of caspase 9. Within the mitochondrial amplification loop, T. gondii significantly inhibited the Fas/CD95-triggered release of cytochrome c into the host cell cytosol. These results indicate that T. gondii inhibits Fas/CD95-mediated apoptosis in type II cells primarily by decreasing the apoptogenic function of mitochondria.     

The essential Schizosaccharomyces pombe cdc23 DNA replication gene shares structural and functional homology with the Saccharomyces cerevisiae DNA43 (MCM10) gene

The fission yeast cdc23 gene is required for correct DNA replication: cdc23 mutants show reduced rates of DNA synthesis and become elongated after cell-cycle arrest. We have cloned the Schizosaccharomyces pombe cdc23 gene by complementation of the temperature-sensitive phenotype of cdc23-M36 and confirmed the identity of the gene by integrative mapping. Analysis of the DNA sequence reveals that cdc23 can encode a protein of 593 amino acids (M_r=67 kDa) with 22% overall identity and many structural homologies with the product of the Saccharomyces cerevisiae DNA43 (MCM10) gene which is required for correct initiation of DNA synthesis at chromosomal origins of replication. Construction of a cdc23 null allele has established that the cdc23 gene is essential for viability, with cdc23 deletion mutant spores germinating but undergoing arrest with undivided nuclei in the first or second cell cycle. The S. pombe cdc23 gene on an expression plasmid is able to complement the S. cerevisiae dna43-1 mutant. These structural and functional homologies between two distantly related species suggest that cdc23 and DNA43 may represent genes for a conserved essential eukaryotic DNA replication function. Received: 12 April / 6 July 1998