Regulated vacuole fusion and fission in Schizosaccharomyces pombe: an osmotic response dependent on MAP kinases.
BACKGROUND
The budding yeast Saccharomyces cerevisiae uses two mitogenactivated protein (MAP) kinase cascades, the Hog1p and the Mpk1p pathways, to signal responses to hypertonic and hypotonic stress, respectively. Mammalian cells and the fission yeast Schizosaccharomyces pombe have functional homologues of Hog1p - p38/RK/CSBP and Sty1 - which, unlike Hog1p, also mediate other responses. We have investigated the involvement of S. pombe MAP kinase pathways in signalling a newly described response to osmotic stress - that of vacuole fusion and fission.


RESULTS
When S. pombe is placed into water, its vacuoles rapidly fuse into larger structures enclosing a greater proportion of the cell's volume. Under some conditions, its vacuoles can slowly fragment in response to salt. Fission requires the Sty1 pathway and also Pmk1, the homologue of S. cerevisiae Mpk1p. Fusion requires Pmk1, Ypt7 - the homologue of a protein involved in S. cerevisiae vacuole fusion - and part of the Sty1 pathway, although Sty1 phosphorylation is unaffected by hypotonic conditions.


CONCLUSIONS
Vacuole fusion and fission appear to be homeostatic mechanisms that restore the concentration of the cytosol. Vacuole fusion, like stimulated secretion in higher eukaryotes, is a rapid and specific process of membrane fusion in response to an external stimulus. The Sty1 pathway, in addition to its role in responding to hypertonic stress, is required at a basal level for the expression of factors required to respond to hypotonic stress - a mechanism that may allow the cell to use a common pathway for different responses.
Bone
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School of Biological Sciences, University of Sussex, Falmer, Brighton, BN1 9QG, UK.
Millar
J B
JB
Toda
T
T
Armstrong
J
J
D000595:Amino Acid Sequence; D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000502:physiology*; D004285:Dogs; D005656:Fungal Proteins / Q000502:physiology*; D019204:GTP-Binding Proteins / Q000502:physiology; D006706:Homeostasis; D006982:Hypertonic Solutions / Q000494:pharmacology*; D007038:Hypotonic Solutions / Q000494:pharmacology*; D007424:Intracellular Fluid / Q000737:chemistry; D008561:Membrane Fusion / Q000187:drug effects / Q000502:physiology*; D020928:Mitogen-Activated Protein Kinases; D008969:Molecular Sequence Data; D009997:Osmotic Pressure; D011494:Protein Kinases / Q000502:physiology*; D012441:Saccharomyces cerevisiae / Q000502:physiology; D029701:Saccharomyces cerevisiae Proteins; D012568:Schizosaccharomyces / Q000187:drug effects / Q000201:enzymology / Q000502:physiology*; D029702:Schizosaccharomyces pombe Proteins; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D014617:Vacuoles / Q000187:drug effects / Q000502:physiology*; D020691:rab GTP-Binding Proteins
D005656:Fungal Proteins; D006982:Hypertonic Solutions; D007038:Hypotonic Solutions; D029701:Saccharomyces cerevisiae Proteins; D029702:Schizosaccharomyces pombe Proteins; D011494:Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; C088589:HOG1 protein, S cerevisiae; D020928:Mitogen-Activated Protein Kinases; C102726:Pmk1 protein, Magnaporthe grisea; C095409:sty1 protein, S pombe; D019204:GTP-Binding Proteins; C079005:YPT7 protein, S cerevisiae; D020691:rab GTP-Binding Proteins