Immunity to K1 killer toxin: internal TOK1 blockade

The Pharmacology of Two-Pore Domain Potassium Channels

Two-pore domain potassium channels are formed by subunits that each contain two pore-loops moieties. Whether the channels are expressed in yeast or the human central nervous system, two subunits come together to form a single potassium selective pore. TOK1, the first two-domain channel was cloned from Saccharomyces cerevisiae in 1995 and soon thereafter, 15 distinct K2P subunits were identified in the human genome. The human K2P channels are stratified into six K2P subfamilies based on sequence as well as physiological or pharmacological similarities. Functional K2P channels pass background (or "leak") K+ currents that shape the membrane potential and excitability of cells in a broad range of tissues. In the years since they were first described, classical functional assays, latterly coupled with state-of-the-art structural and computational studies have revealed the mechanistic basis of K2P channel gating in response to specific physicochemical or pharmacological stimuli. The growing appreciation that K2P channels can play a pivotal role in the pathophysiology of a growing spectrum of diseases makes a compelling case for K2P channels as targets for drug discovery. Here, we summarize recent advances in unraveling the structure, function, and pharmacology of the K2P channels.

TOK channels use the two gates in classical K+ channels to achieve outward rectification

TOKs are outwardly rectifying K⁺ channels in fungi with two pore-loops and eight transmembrane spans. Here, we describe the TOKs from four pathogens that cause the majority of life-threatening fungal infections in humans. These TOKs pass large currents only in the outward direction like the canonical isolate from Saccharomyces cerevisiae (ScTOK), and distinct from other K⁺ channels. ScTOK, AfTOK1 (Aspergillus fumigatus), and H99TOK (Cryptococcus neoformans grubii) are K⁺ -selective and pass current above the K⁺ reversal potential. CaTOK (Candida albicans) and CnTOK (Cryptococcus neoformans neoformans) pass both K⁺ and Na⁺ and conduct above a reversal potential reflecting the mixed permeability of their selectivity filter. Mutations in CaTOK and ScTOK at sites homologous to those that open the internal gates in classical K⁺ channels are shown to produce inward TOK currents. A favored model for outward rectification is proposed whereby the reversal potential determines ion occupancy, and thus, conductivity, of the selectivity filter gate that is coupled to an imperfectly restrictive internal gate, permitting the filter to sample ion concentrations on both sides of the membrane.

Expression of K1 Toxin Derivatives in Saccharomyces cerevisiae Mimics Treatment with Exogenous Toxin and Provides a Useful Tool for Elucidating K1 Mechanisms of Action and Immunity

Killer toxin K1 is a heterodimeric protein toxin secreted by Saccharomyces cerevisiae strains infected with the M1 double-stranded RNA ‘killer’ virus. After binding to a primary receptor at the level of the cell wall, K1 interacts with its secondary plasma membrane receptor Kre1p, eventually leading to an ionophoric disruption of membrane function. Although it has been under investigation for decades, neither the particular mechanisms leading to toxicity nor those leading to immunity have been elucidated. In this study, we constructed derivatives of the K1α subunit and expressed them in sensitive yeast cells. We show that these derivatives are able to mimic the action of externally applied K1 toxin in terms of growth inhibition and pore formation within the membrane, leading to a suicidal phenotype that could be abolished by co-expression of the toxin precursor, confirming a mechanistic similarity of external and internal toxin action. The derivatives were successfully used to investigate a null mutant completely resistant to externally applied toxin. They provide a valuable tool for the identification of so far unknown gene products involved in K1 toxin action and/or immunity.

Alkali metal cation transport and homeostasis in yeasts

The maintenance of appropriate intracellular concentrations of alkali metal cations, principally K(+) and Na(+), is of utmost importance for living cells, since they determine cell volume, intracellular pH, and potential across the plasma membrane, among other important cellular parameters. Yeasts have developed a number of strategies to adapt to large variations in the concentrations of these cations in the environment, basically by controlling transport processes. Plasma membrane high-affinity K(+) transporters allow intracellular accumulation of this cation even when it is scarce in the environment. Exposure to high concentrations of Na(+) can be tolerated due to the existence of an Na(+), K(+)-ATPase and an Na(+), K(+)/H(+)-antiporter, which contribute to the potassium balance as well. Cations can also be sequestered through various antiporters into intracellular organelles, such as the vacuole. Although some uncertainties still persist, the nature of the major structural components responsible for alkali metal cation fluxes across yeast membranes has been defined within the last 20 years. In contrast, the regulatory components and their interactions are, in many cases, still unclear. Conserved signaling pathways (e.g., calcineurin and HOG) are known to participate in the regulation of influx and efflux processes at the plasma membrane level, even though the molecular details are obscure. Similarly, very little is known about the regulation of organellar transport and homeostasis of alkali metal cations. The aim of this review is to provide a comprehensive and up-to-date vision of the mechanisms responsible for alkali metal cation transport and their regulation in the model yeast Saccharomyces cerevisiae and to establish, when possible, comparisons with other yeasts and higher plants.

The coevolution of cooperation and dispersal in social groups and its implications for the emergence of multicellularity

BACKGROUND

Recent work on the complexity of life highlights the roles played by evolutionary forces at different levels of individuality. One of the central puzzles in explaining transitions in individuality for entities ranging from complex cells, to multicellular organisms and societies, is how different autonomous units relinquish control over their functions to others in the group. In addition to the necessity of reducing conflict over effecting specialized tasks, differentiating groups must control the exploitation of the commons, or else be out-competed by more fit groups.

RESULTS

We propose that two forms of conflict - access to resources within groups and representation in germ line - may be resolved in tandem through individual and group-level selective effects. Specifically, we employ an optimization model to show the conditions under which different within-group social behaviors (cooperators producing a public good or cheaters exploiting the public good) may be selected to disperse, thereby not affecting the commons and functioning as germ line. We find that partial or complete dispersal specialization of cheaters is a general outcome. The propensity for cheaters to disperse is highest with intermediate benefit:cost ratios of cooperative acts and with high relatedness. An examination of a range of real biological systems tends to support our theory, although additional study is required to provide robust tests.

CONCLUSION

We suggest that trait linkage between dispersal and cheating should be operative regardless of whether groups ever achieve higher levels of individuality, because individual selection will always tend to increase exploitation, and stronger group structure will tend to increase overall cooperation through kin selected benefits. Cheater specialization as dispersers offers simultaneous solutions to the evolution of cooperation in social groups and the origin of specialization of germ and soma in multicellular organisms.

The antiproliferative role of ERG K+ channels in rat osteoblastic cells

We report on the role of K+ currents in the mechanisms regulating the proliferation of UMR 106-01 osteoblastic osteosarcoma cells. Electrophysiological analysis showed that UMR 106-01 cells produce robust K+ currents that can be pharmacologically separated into two major components: a E-4031-susceptible current, I E-4031, and a tetraethylammonium (TEA)-susceptible component, I TEA. Western blot and RT-PCR analysis showed that I E-4031 is produced by ether a go-go (eag)-related channels (ERG). Incubation of the cells with E-4031 enhanced their proliferation by 80%. Application of E-4031 in the bath solution did not induce instantaneous changes in the membrane resting potential or in the level of cytosolic calcium; however, the cells were slightly depolarized and the calcium content was significantly increased upon prolonged incubation with the compound. Taken together these findings indicate that ERG channels can impair cell proliferation. This is a novel finding that underscores new modes of regulation of mitosis by voltage-gated K+ channels and provides an unexpected insight into the current view of the mechanisms governing bone tissue proliferation.

Yeast viral killer toxins: lethality and self-protection

Since the discovery of toxin-secreting killer yeasts more than 40 years ago, research into this phenomenon has provided insights into eukaryotic cell biology and virus-host-cell interactions. This review focuses on the most recent advances in our understanding of the basic biology of virus-carrying killer yeasts, in particular the toxin-encoding killer viruses, and the intracellular processing, maturation and toxicity of the viral protein toxins. The strategy of using eukaryotic viral toxins to effectively penetrate and eventually kill a eukaryotic target cell will be discussed, and the cellular mechanisms of self-defence and protective immunity will also be addressed.

Dissecting toxin immunity in virus-infected killer yeast uncovers an intrinsic strategy of self-protection

Toxin-secreting "killer" yeasts were initially identified >40 years ago in Saccharomyces cerevisiae strains infected with a double-stranded RNA "killer" virus. Despite extensive research conducted on yeast killer toxins, the mechanism of protecting immunity by which toxin-producing cells evade the lethal activities of these proteins has remained elusive. Here, we identify the mechanism leading to protecting immunity in a killer yeast secreting a viral alpha/beta protein toxin (K28) that enters susceptible cells by receptor-mediated endocytosis and, after retrograde transport into the cytosol, blocks DNA synthesis, resulting in both cell-cycle arrest and caspase-mediated apoptosis. We demonstrate that toxin immunity is effected within the cytosol of a toxin-secreting yeast and occurs via the formation of complexes between reinternalized toxin and unprocessed precursor moieties that are subsequently ubiquitinated and proteasomally degraded, eliminating the active form of the toxin. Interference with cellular ubiquitin homeostasis, either through overexpression of mutated ubiquitin (Ub-RR(48/63)) or by blocking deubiquitination, prevents ubiquitination of toxin and results in an impaired immunity and the expression of a suicidal phenotype. The results presented here reveal the uniquely elegant and efficient strategy that killer cells have developed to circumvent the lethal effects of the toxin they produce.

RNA viruses redirect host factors to better amplify their genome

This chapter provides an updated view of the host factors that are, at present, believed to participate in replication/transcription of RNA viruses. One of the major hurdles faced when attempting to identify host factors specifically involved in viral RNA replication/transcription is how to discriminate these factors from those involved in translation. Several of the host factors shown to affect viral RNA synthesis are factors known to be involved in protein synthesis, for example, translation factors. In addition, some of the factors identified to date appear to influence viral RNA amplification as well as viral protein synthesis, and translation and replication are frequently tightly associated. Several specific host factors actively participating in viral RNA transcription/replication have been identified and the regions of host protein/replicase or host protein/viral RNA interaction have been determined. The chapter centers exclusively on those factors that appear functionally important for viral amplification. It presents a list of the viruses for which a specific host factor associates with the polymerase, affecting viral genome amplification. It also indicates the usually accepted cell function of the factor and the viral polymerase or polymerase subunit to which the host factor binds.

Viruses activate a genetically conserved cell death pathway in a unicellular organism

Given the importance of apoptosis in the pathogenesis of virus infections in mammals, we investigated the possibility that unicellular organisms also respond to viral pathogens by activating programmed cell death. The M1 and M2 killer viruses of Saccharomyces cerevisiae encode pore-forming toxins that were assumed to kill uninfected yeast cells by a nonprogrammed assault. However, we found that yeast persistently infected with these killer viruses induce a programmed suicide pathway in uninfected (nonself) yeast. The M1 virus-encoded K1 toxin is primarily but not solely responsible for triggering the death pathway. Cell death is mediated by the mitochondrial fission factor Dnm1/Drp1, the K+ channel Tok1, and the yeast metacaspase Yca1/Mca1 encoded by the target cell and conserved in mammals. In contrast, cell death is inhibited by yeast Fis1, a pore-forming outer mitochondrial membrane protein. This virus-host relationship in yeast resembles that of pathogenic human viruses that persist in their infected host cells but trigger programmed death of uninfected cells.

Strategies of microbial cheater control

The potential benefits of cooperation in microorganisms can be undermined by genetic conflict within social groups, which can take the form of 'cheating'. For cooperation to succeed as an evolutionary strategy, the negative effects of such conflict must somehow be either prevented or mitigated. To generate an interpretive framework for future research in microbial behavioural ecology, here we outline a wide range of hypothetical mechanisms by which cheaters might be constrained.

Killing of Candida albicans by human salivary histatin 5 is modulated, but not determined, by the potassium channel TOK1

Salivary histatin 5 (Hst 5), a potent toxin for the human fungal pathogen Candida albicans, induces noncytolytic efflux of cellular ATP, potassium, and magnesium in the absence of cytolysis, implicating these ion movements in the toxin's fungicidal activity. Hst 5 action on Candida resembles, in many respects, the action of the K1 killer toxin on Saccharomyces cerevisiae, and in that system the yeast plasma membrane potassium channel, Tok1p, has recently been reported to be a primary target of toxin action. The question of whether the Candida homologue of Saccharomyces Tok1p might be a primary target of Hst 5 action has now been investigated by disruption of the C. albicans TOK1 gene. The resultant strains (TOK1/tok1) and (tok1/tok1) were compared with wild-type Candida (TOK1/TOK1) for relative ATP leakage and killing in response to Hst 5. Patch-clamp measurements on Candida protoplasts were used to verify the functional deletion of Tok1p and to provide its first description in Candida. Tok1p is an outwardly rectifying, noisily gated, 40-pS channel, very similar to that described in Saccharomyces. Knockout of CaTOK1 (tok1/tok1) completely abolishes the currents and gating events characteristic of Tok1p. Also, knockout (tok1/tok1) increases residual viability of Candida after Hst 5 treatment to 27%, from 7% in the wild type, while the single allele deletion (TOK1/tok1) increases viability to 18%. Comparable results were obtained for Hst-induced ATP efflux, but quantitative features of ATP loss suggest that wild-type TOK1 genes function cooperatively. Overall, very substantial killing and ATP efflux are produced by Hst 5 treatment after complete knockout of wild-type TOK1, making clear that Tok1p channels are not the primary site of Hst 5 action, even though they do play a modulating role.

Hyperpolarization moves S4 sensors inward to open MVP, a methanococcal voltage-gated potassium channel

MVP, a Methanococcus jannaschii voltage-gated potassium channel, was cloned and shown to operate in eukaryotic and prokaryotic cells. Like pacemaker channels, MVP opens on hyperpolarization using S4 voltage sensors like those in classical channels activated by depolarization. The MVP S4 span resembles classical sensors in sequence, charge, topology and movement, traveling inward on hyperpolarization and outward on depolarization (via canaliculi in the protein that bring the extracellular and internal solutions into proximity across a short barrier). Thus, MVP opens with sensors inward indicating a reversal of S4 position and pore state compared to classical channels. Homologous channels in mammals and plants are expected to function similarly.

Characterization of potassium transport in wild-type and isogenic yeast strains carrying all combinations of trk1, trk2 and tok1 null mutations

Saccharomyces cerevisiae cells express three defined potassium-specific transport systems en-coded by TRK1, TRK2 and TOK1. To gain a more complete understanding of the physiological function of these transport proteins, we have constructed a set of isogenic yeast strains carrying all combinations of trk1delta, trk2delta and tok1delta null mutations. The in vivo K+ transport characteristics of each strain have been documented using growth-based assays, and the in vitro biochemical and electrophysiological properties associated with K+ transport have been determined. As has been reported previously, Trk1p and Trk2p facilitate high-affinity potassium uptake and appear to be functionally redundant under a wide range of environmental conditions. In the absence of TRK1 and TRK2, strains lack the ability specifically to take up K+, and trk1deltatrk2delta double mutant cells depend upon poorly understood non-specific cation uptake mechanisms for growth. Under conditions that impair the activity of the non-specific uptake system, termed NSC1, we have found that the presence of functional Tok1p renders cells sensitive to Cs+. Based on this finding, we have established a growth-based assay that monitors the in vivo activity of Tok1p.

Kre1p, the plasma membrane receptor for the yeast K1 viral toxin

Saccharomyces cerevisiae K1 killer strains are infected by the M1 double-stranded RNA virus encoding a secreted protein toxin that kills sensitive cells by disrupting cytoplasmic membrane function. Toxin binding to spheroplasts is mediated by Kre1p, a cell wall protein initially attached to the plasma membrane by its C-terminal GPI anchor. Kre1p binds toxin directly. Both cells and spheroplasts of Deltakre1 mutants are completely toxin resistant; binding to cell walls and spheroplasts is reduced to 10% and < 0.5%, respectively. Expression of K28-Kre1p, an inactive C-terminal fragment of Kre1p retaining its toxin affinity and membrane anchor, fully restored toxin binding and sensitivity to spheroplasts, while intact cells remained resistant. Kre1p is apparently the toxin membrane receptor required for subsequent lethal ion channel formation.

Carboxyl tail prevents yeast K(+) channel closure: proposal of an integrated model of TOK1 gating

TOK1 encodes the channel responsible for the prominent outward K(+) current of the yeast plasma membrane. It can dwell in several impermeable states, including a rapidly transiting, K(+)-electromotive-force-dependent "R" (rectifying) state, a voltage-independent "IB" (interburst) state, and a set of [K(+)](ext) and voltage-dependent "C" (closed) states. Whereas evidence suggests that the C states result from the constriction of an inner gate at the cytosolic end of the pore, R is most likely an intrinsic gating property of the K(+) filter. Here, we present evidence that Tok1's carboxyl-tail domain also plays an intimate role in channel gating by dynamically preventing inner-gate closures. We present an integrated model of TOK1 gating in which the filter gate, inner gate, and carboxyl tail interact to produce the various phenomenological states. Both wild-type and tailless behaviors can be replicated using Monte Carlo computer simulations based on this model.