In electrically nonexcitable cells, Ca2+ influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and prolifera...

Store-operated calcium channels.

Conformational diseases are caused by mutations altering the folding pathway or final conformation of a protein. Many conformational diseases are caused by mutations in secretory proteins and reach from metabolic diseases, e.g. diabetes, to developmental and neurological diseases, e.g. Alzheimer's disease. Expression of mutant proteins disrupts protein folding in the endoplasmic reticulum (ER), causes ER stress, and activates a signaling network called the unfolded protein response (UPR). The UPR increases the biosynthetic capacity of the secretory pathway through upregulation of ER chaperone and foldase expression. In addition, the UPR decreases the biosynthetic burden of the secretory pathway by downregulating expression of genes encoding secreted proteins. Here we review our current understanding of how an unfolded protein signal is generated, sensed, transmitted across the ER membrane, and how downstream events in this stress response are regulated. We propose a model in which the activity of UPR signaling pathways reflects the biosynthetic activity of the ER. We summarize data that shows that this information is integrated into control of cellular events, which were previously not considered to be under control of ER signaling pathways, e.g. execution of differentiation and starvation programs.

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ER stress and the unfolded protein response.

Isogenic strain construction and gene mapping in Candida albicans.

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

Osmotic Stress Signaling and Osmoadaptation in Yeasts

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca2+ signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.

Cell Wall Integrity Signaling in Saccharomyces cerevisiae

Depletion of calcium ions (Ca 2+ ) from the endoplasmic reticulum (ER) of yeast cells resulted in the activation of the unfolded protein response (UPR) signaling pathway involving Ire1p and Hac1p. The depleted ER also stimulated Ca 2+ influx at the plasma membrane through the Cch1p–Mid1p Ca 2+ channel and another system. Surprisingly, both Ca 2+ influx systems were stimulated upon accumulation of misfolded proteins in the ER even in the presence of Ca 2+ . The ability of misfolded ER proteins to stimulate Ca 2+ influx at the plasma membrane did not require Ire1p or Hac1p, and Ca 2+ influx and signaling factors were not required for initial UPR signaling. However, activation of the Ca 2+ channel, calmodulin, calcineurin and other factors was necessary for long‐term survival of cells undergoing ER stress. A similar calcium cell survival (CCS) pathway operates in the pathogenic fungi and promotes resistance to azole antifungal drugs. These findings reveal an unanticipated new regulatory mechanism that couples ER stress to Ca 2+ influx and signaling pathways, which help to prevent cell death and promote resistance to an important class of fungistatic drugs.

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Essential role of calcineurin in response to endoplasmic reticulum stress

refill Ca 21 stores within the secretory pathway. Mutants lacking Pmr1p, a conserved Ca 21 pump in the secretory pathway, exhibit higher rates of Ca 21 influx relative to wild-type cells due to the stimulation of a high-affinity Ca 21 uptake system. Stimulation of this Ca 21 uptake system was blocked in pmr1 mutants by expression of mammalian SERCA pumps. The high-affinity Ca 21 uptake system was also stimulated in wild-type cells overexpressing vacuolar Ca 21 transporters that competed with Pmr1p for substrate. A screen for yeast mutants specifically defective in the high-affinity Ca 21 uptake system revealed two genes, CCH1 and MID1, previously implicated in Ca 21 influx in response to mating pheromones. Cch1p and Mid1p were localized to the plasma membrane, coimmunoprecipitated from solubilized membranes, and shown to function together within a single pathway that ensures that adequate levels of Ca 21 are supplied to Pmr1p to sustain secretion and growth. Expression of Cch1p and Mid1p was not affected in pmr1 mutants. The evidence supports the hypothesis that yeast maintains a homeostatic mechanism related to CCE in mammalian cells. The homology between Cch1p and the catalytic subunit of voltage-gated Ca 21 channels raises the possibility that in some circumstances CCE in animal cells may involve homologs of Cch1p and a conserved regulatory mechanism.

A Homolog of Voltage-Gated Ca2+ Channels Stimulated by Depletion of Secretory Ca2+ in Yeast

Endoplasmic reticulum (ER) stress in the budding yeast Saccharomyces cerevisiae triggers Ca2+ influx through a plasma membrane channel composed of Cch1 and Mid1. This response activates calcineurin, which helps to prevent cell death during multiple forms of ER stress, including the response to azole-class antifungal drugs. Herein, we show that ER stress activates the cell integrity mitogen-activate protein kinase cascade in yeast and that the activation of Pkc1 and Mpk1 is necessary for stimulation of the Cch1-Mid1 Ca2+ channel independent of many known targets of Mpk1 (Rlm1, Swi4, Swi6, Mih1, Hsl1, and Swe1). ER stress generated in response to miconazole, tunicamycin, or other inhibitors also triggered a transient G2/M arrest that depended upon the Swe1 protein kinase. Calcineurin played little role in the Swe1-dependent cell cycle arrest and Swe1 had little effect on calcineurin-dependent avoidance of cell death. These findings help to clarify the interactions between Mpk1, calcineurin, and Swe1 and suggest that the calcium cell survival pathway promotes drug resistance independent of both the unfolded protein response and the G2/M cell cycle checkpoint.

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Mitogen-activated Protein Kinase Stimulation of Ca2+ Signaling Is Required for Survival of Endoplasmic Reticulum Stress in Yeast

Because cytoplasmic dynein plays numerous critical roles in eukaryotic cells, determining the subunit composition and the organization and functions of the subunits within dynein are important goals. This has been difficult partly because of accessory polypeptide heterogeneity of dynein populations. The motor domain containing heavy chains of cytoplasmic dynein are associated with multiple intermediate, light intermediate, and light chain accessory polypeptides. We examined the organization of these subunits within cytoplasmic dynein by separating the molecule into two distinct subcomplexes. These subcomplexes were competent to reassemble into a molecule with dynein-like properties. One subcomplex was composed of the dynein heavy and light intermediate chains whereas the other subcomplex was composed of the intermediate and light chains. The intermediate and light chain subcomplex could be further separated into two pools, only one of which contained dynein light chains. The two pools had distinct intermediate chain compositions, suggesting that intermediate chain isoforms have different light chain–binding properties. When the two intermediate chain pools were characterized by analytical velocity sedimentation, at least four molecular components were seen: intermediate chain monomers, intermediate chain dimers, intermediate chain monomers with bound light chains, and a mixture of intermediate chain dimers with assorted bound light chains. These data provide new insights into the compositional heterogeneity and assembly of the cytoplasmic dynein complex and suggest that individual dynein molecules have distinct molecular compositions in vivo.

Subunit organization in cytoplasmic dynein subcomplexes

Calcium is one of the most ubiquitous second messengers, in addition to being a fundamentally important cofactor for many proteins' functions. Thus, cells from fungi to plants to animals have evolved mechanisms for carefully controlling calcium concentrations in their organelles and cytosol. This perspective discusses the similarities and differences between yeast and animals in cation channels of two families: the transient receptor potential channels (TRPCs) and voltage-gated calcium channels (VGCCs). One of the key differences highlighted is in the channels implicated in the response of yeast and animal cells to depletion of calcium from intracellular stores, and the differences in localization of these channels between yeast and animal cells. Understanding the logic of fungal Ca(2+) channels, therefore, may provide new insights into the organization and regulation of cellular calcium signaling networks in animals.

https://stke.sciencemag.org/content/sigtrans/2002/127/pe17.full.pdf

Myriam Bonilla

Papers

Essential role of calcineurin in response to endoplasmic reticulum stress

A Homolog of Voltage-Gated Ca2+ Channels Stimulated by Depletion of Secretory Ca2+ in Yeast

Mitogen-activated Protein Kinase Stimulation of Ca2+ Signaling Is Required for Survival of Endoplasmic Reticulum Stress in Yeast

Subunit organization in cytoplasmic dynein subcomplexes

Calcium Release and Influx in Yeast: TRPC and VGCC Rule Another Kingdom