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

Calcineurin is a eukaryotic Ca2+- and calmodulin-dependent serine/threonine protein phosphatase. It is a heterodimeric protein consisting of a catalytic subunit calcineurin A, which contains an act...

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.4.1483

Calcineurin : form and function

Serine/Threonine Phosphatases: Mechanism through Structure

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

The view of the lysosome as the terminal end of cellular catabolic pathways has been challenged by recent studies showing a central role of this organelle in the control of cell function. Here we show that a lysosomal Ca2+ signalling mechanism controls the activities of the phosphatase calcineurin and of its substrate ​TFEB, a master transcriptional regulator of lysosomal biogenesis and autophagy. Lysosomal Ca2+ release through ​mucolipin 1 (​MCOLN1) activates calcineurin, which binds and dephosphorylates ​TFEB, thus promoting its nuclear translocation. Genetic and pharmacological inhibition of calcineurin suppressed ​TFEB activity during starvation and physical exercise, while calcineurin overexpression and constitutive activation had the opposite effect. Induction of autophagy and lysosomal biogenesis through ​TFEB required ​MCOLN1-mediated calcineurin activation. These data link lysosomal calcium signalling to both calcineurin regulation and autophagy induction and identify the lysosome as a hub for the signalling pathways that regulate cellular homeostasis.

/pdf/lysosomal-calcium-signalling-regulates-autophagy-through-38bzt2y1tu.pdf

Lysosomal calcium signalling regulates autophagy through calcineurin and TFEB

Recombinant forms of the A and B subunits of the protein phosphatase calcineurin were produced in Escherichia coli, reconstituted into a heterodimer and purified to homogeneity. The reconstituted heterodimer exhibited properties like that of bovine brain calcineurin. This included calmodulin-stimulated activity and a subunit stoichiometry and Stokes radius consistent with native-like structure. In order to map the region on the A subunit where calcineurin B binds, a series of overlapping 20-residue peptides corresponding to this putative domain were synthesized. Using isolated calcineurin A and B subunits, an assay that relied upon peptide inhibition of calcineurin B stimulation of calcineurin A activity was developed. All five peptides, but not a control peptide, inhibited calcineurin B-dependent stimulation of calcineurin A although with different potencies. The three most effective inhibitory peptides spanned calcineurin A residues 338-377. These three peptides also altered the electrophoretic mobility of the isolated calcineurin B subunit during native polyacrylamide gel electrophoresis indicating a direct interaction between these peptides and calcineurin B. The peptide corresponding to residues 348-367 was also able to block binding of calcineurin B to the catalytic subunit.

Calcineurin subunit interactions: Mapping the calcineurin B binding domain on calcineurin A

The catalytic reaction of the Mn2+ form of the native bacteriophage λ phosphatase and the H76N mutant was studied with the substrate p-nitrophenyl phosphate using heavy atom isotope effects and pH-...

The transition state of the phosphoryl-transfer reaction catalyzed by the lambda ser/thr protein phosphatase

Regulation of the protein phosphatase calcineurin by redox: implications for catalysis and signal transduction

The authors declare no conflicts of interest. 

Pamela Mertz

Papers

Calcineurin : form and function

Calcineurin subunit interactions: Mapping the calcineurin B binding domain on calcineurin A

The transition state of the phosphoryl-transfer reaction catalyzed by the lambda ser/thr protein phosphatase

Regulation of the protein phosphatase calcineurin by redox: implications for catalysis and signal transduction

Themes in molecular life science education: Engaging pedagogies and faculty development for student success.