Viral Induced Yeast Apoptosis ⁎ Manfred J
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Available online at www.sciencedirect.com Biochimica et Biophysica Acta 1783 (2008) 1413–1417 www.elsevier.com/locate/bbamcr Review Viral induced yeast apoptosis ⁎ Manfred J. Schmitt , Jochen Reiter Molecular and Cell Biology, FR 8.3 Biosciences, Building A 1.5, Saarland University, D-66041 Saarbrücken, Germany Received 4 October 2007; received in revised form 17 January 2008; accepted 18 January 2008 Available online 7 February 2008 Abstract In an analogous system to mammals, induction of an apoptotic cell death programme (PCD) in yeast is not only restricted to various exogenous factors and stimuli, but can also be triggered by viral killer toxins and viral pathogens. In yeast, toxin secreting killer strains are frequently infected with double-stranded (ds)RNA viruses that are responsible for killer phenotype expression and toxin secretion in the infected host. In most cases, the viral toxins are either pore-forming proteins (such as K1, K2, and zygocin) that kill non-infected and sensitive yeast cells by disrupting cytoplasmic membrane function, or protein toxins (such as K28) that act in the nucleus by blocking DNA synthesis and subsequently causing a G1/S cell cycle arrest. Interestingly, while all these virus toxins cause necrotic cell death at high concentration, they trigger caspase- and ROS-mediated apoptosis at low-to-moderate concentration, indicating that even low toxin doses are deadly by triggering PCD in enemy cells. Remarkably, viral toxins are not solely responsible for cell death induction in vivo, as killer viruses themselves were shown to trigger apoptosis in non-infected yeast. Thus, as killer virus-infected and toxin secreting yeasts are effectively protected and immune to their own toxin, killer yeasts bear the intrinsic potential to dominate over time in their natural habitat. © 2008 Elsevier B.V. All rights reserved. Keywords: Yeast; Apoptosis; Killer toxin; dsRNA virus 1. Introduction [2,4,24,29]. In the yeast Saccharomyces cerevisiae, three different killer toxins have been identified so far (K1, K2, K28), all of In higher eukaryotes and mammals, virus infections are known which are genetically encoded by cytoplasmic persisting double- to induce a programmed cell death (PCD) pathway as first line of stranded RNA viruses encoding an unprocessed precursor of a defense in preventing virus transmission. Most remarkably, secreted α/β protein toxin [34,40]. Most viral killer toxins, like the unicellular organisms such as yeast likewise respond to viral S. cerevisiae K1 toxin and the Zygosaccharomyces bailii toxin pathogens and virus toxins, indicating that PCD is evolutionarily zygocin, act as ionophores and disrupt cytoplasmic membrane conserved and most likely arose before multicellular organisms function by forming cation-specific plasma membrane pores have developed [15,27]. Interestingly, the onset of yeast apoptosis [25,37,38]. In contrast however, the S. cerevisiae K28 toxin enters was recently shown to also have an effect on the assembly- susceptible cells by receptor-mediated endocytosis, travels the dependent maturation of insect tetraviral procapsids in vivo [35]. secretion pathway in reverse, and finally acts in the nucleus by In yeast, the production of virally encoded, cytotoxic proteins irreversibly blocking DNA synthesis and causing a G1/S cell (so-called killer toxins) is a widespread phenomenon and typically cycle arrest [7,13,31]. associated with the secretion of protein toxins that kill susceptible In higher multicellular organisms it is well known that pore- target (yeast) cells in a two-step receptor-mediated fashion forming toxins like Staphylococcus aureus alpha toxin and/or inhibitors of protein synthesis like diphtheria toxin produced and secreted by Corynebacterium diphtheriae induce apoptosis [39]. The initial finding of cell death with apoptosis-like fea- ⁎ Corresponding author. Molecular and Cell Biology (FR 8.3 Biosciences), Saarland University, PO Box 15 11 50, Campus Building A 1.5, D-66041 tures in unicellular yeast [21] was unexpected and it was Saarbrücken, Germany. Tel.: +49 681 302 4730; fax: +49 681 302 4710. doubted that single-cell organisms would have any advantage in E-mail address: [email protected] (M.J. Schmitt). committing suicide. However, intensive studies of apoptotic 0167-4889/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.bbamcr.2008.01.017 1414 M.J. Schmitt, J. Reiter / Biochimica et Biophysica Acta 1783 (2008) 1413–1417 phenomena in S. cerevisiae demonstrated that programmed cell 2. Killer yeasts and viral killer toxins death in yeast can be induced by a great number of exogenous and intrinsic stresses such as H2O2, UV-irradiation, acetic dsRNA viruses in yeast were initially discovered as deter- acid, cell aging, high pheromone concentration, defects in pro- minants of the killer phenomenon and are now known to be tein N-glycosylation, antimicrobial peptides, and many more associated with the presence of cytoplasmically inherited my- scenarios [6,12,14,17,20,22,26,32]. In the majority of cases coviruses, which are frequently found in different yeast genera. describing an apoptotic phenotype in yeast it was shown that Among these, killers of S. cerevisiae, Z. bailii, Hanseniaspora debilitated cells die for the benefit of the whole yeast population uvarum and Ustilago maydis – the latter being the cause of corn saving limited nutrients for healthy cells in order to enable smut – have been most intensively studied. Characteristic for survival of the whole population [9,14]. Similar to mammalian all killers is the secretion of a particular protein toxin that is lethal apoptosis, reactive oxygen species (ROS) and mitochondria are to sensitive strains of different species and genera. Cell killing is central to most of these scenarios, and the striking similarity usually achieved in a receptor-mediated process, requiring initial between yeast and mammalian apoptosis was further underlined toxin binding to receptors within the outer yeast cell surface by the finding of yeast orthologues of mammalian pro- and anti- (such as β-1,6-D-glucan, α-1,3-mannoprotein or chitin) and apoptotic factors such as the metacaspase Yca1p [23], the subsequent transfer to a plasma membrane receptor. Depending HtrA2/Omi homologue Nma111p [8], an inhibitor-of-apoptosis on the type of toxin, lethality can either be caused by plasma (IAP) [36], the transkingdom Bax inhibitor-1 protein Bir1p [5] membrane damage, G1/S cell cycle arrest and/or rapid inhibition and two AIF/AMID homologues, the pro-apoptotic serine of DNA synthesis [30] (Fig. 1). protease Aif1p and Ndi1p [19,41]. However, despite all these In S. cerevisiae, Z. bailii and H. uvarum as well as in the maize proteins were shown to function in yeast apoptosis under certain smut fungus U. maydis, the killer phenotype is cytoplasmically conditions, natural pathways in which these proteins are es- inherited and caused by an infection with dsRNA viruses of the sentially involved have not yet been identified, and target(s) of family Totiviridae that are widely distributed among yeasts and yeast metacaspase Yca1p still remain to be elucidated [11]. higher fungi. Since the majority of fungal viruses are non- Fig. 1. Receptor-mediated toxicity of yeast viral killer toxins K28, K1 and zygocin. Killing of a sensitive target (yeast) cell by each virus toxin is envisaged in a two- step process involving initial toxin binding to receptors within the cell wall (R1) and the cytoplasmic membrane (R2). After interaction with the plasma membrane, ionophoric toxins such as K1 and zygocin disrupt plasma membrane integrity, while K28 enters the cell by endocytosis and diffuses into the nucleus to cause non-PCD [note that the cell surface receptors R1 and R2 are different in all three toxins; see also table inset]. At high dose of K28 toxin (N10 pM), sensitive cells arrest in the cell cycle with pre-replicated DNA (1n; left panel), while low concentrations of K28 (b1 pM) activate a programmed cell death pathway exhibiting typical apoptotic markers such as chromosomal DNA fragmentation (TUNEL positive cells), accumulation of reactive oxygen species (ROS) and phosphatidylserine exposure at the external surface of the plasma membrane detected by annexin V staining (mid panel). Host cell proteins involved in toxin-mediated PCD (Kre1p, Tok1p, Yca1p, Dnm1p) or non-PCD (Kre1p) are indicated (the model is based on data from Reiter et al. [27] and Ivanovska and Hardwick [15]). M.J. Schmitt, J. Reiter / Biochimica et Biophysica Acta 1783 (2008) 1413–1417 1415 infectious and symptomless in the corresponding host, they are typically accompanied by DNA fragmentation, chromatin con- often classified as cryptic viruses. All known fungal viruses densation and phosphatidylserine externalization (Fig. 1). This spread vertically by cell-to-cell mating and/or heterokaryon for- process is mediated through yeast caspase Yca1p and the mation. In S. cerevisiae, diploids formed by mating of a killer with generation of ROS [27]. In contrast, high toxin concentrations a sensitive strain are likewise killers, as are all haploid progeny (N10 pM) induce non-apoptotic (necrotic) cell death indepen- of subsequent meiosis. In contrast, non-infected and virus-free dent of Yca1p and ROS. Therefore, killer toxin action can trigger strains are usually sensitive non-killers, while those containing a two modes of cell death: at high concentration, toxin-induced toxin-encoding killer virus and a helper virus (the latter providing apoptosis plays a minor role, while low-to-moderate toxin doses both viruses with Gag and Gag/Pol for viral RNA replication and trigger PCD in susceptible target cells. Since low toxin concen- packaging) survive mating with killers, and cytoplasmic mixing trations resemble the in vivo situation in the natural habitat, of the multiple killer viruses during zygosis accounts for the toxin-induced apoptotic host cell responses seem to be inheritance pattern during meiosis.