Depletion of calcium ions (Ca2+) 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 Ca2+ influx at the plasma membrane through the Cch1p-Mid1p Ca2+ channel and another system. Surprisingly, both Ca2+ influx systems were stimulated upon accumulation of misfolded proteins in the ER even in the presence of Ca2+. The ability of misfolded ER proteins to stimulate Ca2+ influx at the plasma membrane did not require Ire1p or Hac1p, and Ca2+ influx and signaling factors were not required for initial UPR signaling. However, activation of the Ca2+ 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 Ca2+ influx and signaling pathways, which help to prevent cell death and promote resistance to an important class of fungistatic drugs.

No abstract

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.

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.

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.