Hsp90 interaction with Cdc2 and Plo1 kinases contributes to actomyosin ring condensation in fission yeast

Distinct spatiotemporal distribution of Hsp90 under high-heat and mild-heat stress conditions in fission yeast

The molecular chaperone Hsp90 is highly conserved from bacteria to mammals. In fission yeast, Hsp90 is essential in many cellular processes and its expression is known to be increased by heat stress (HS). Here, we describe the distinct spatiotemporal distribution of Hsp90 under high-heat stress (HHS: 45˚C) and mild-heat stress (MHS: 37˚C). Hsp90 is largely distributed in the cytoplasm under non-stressed conditions (27˚C). Under HHS, Hsp90 forms several cytoplasmic granules within 5 minutes, then the granules disappear within 60 minutes. Under MHS, Hsp90 forms fewer granules than under HHS within 5 minutes and strikingly the granules persist and grow in size. In addition, nuclear enrichment of Hsp90 was observed after 60 minutes under both HS conditions. Our data suggest that assembly/disassembly of Hsp90 granules is differentially regulated by temperatures.

Screening Molecular Chaperones Similar to Small Heat Shock Proteins in Schizosaccharomyces pombe

To screen molecular chaperones similar to small heat shock proteins (sHsps), but without α-crystalline domain, heat-stable proteins from Schizosaccharomyces pombe were analyzed by 2-dimensional electrophoresis and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Sixteen proteins were identified, and four recombinant proteins, including cofilin, NTF2, pyridoxin biosynthesis protein (Snz1) and Wos2 that has an α-crystalline domain, were purified. Among these proteins, only Snz1 showed the anti-aggregation activity against thermal denaturation of citrate synthase. However, pre-heating of NTF2 and Wos2 at 70℃ for 30 min, efficiently prevented thermal aggregation of citrate synthase. These results indicate that Snz1 and NTF2 possess molecular chaperone activity similar to sHsps, even though there is no α-crystalline domain in their sequences.

Proteome-wide search for PP2A substrates in fission yeast

PP2A (protein phosphatase 2A) is a major phosphatase in eukaryotic cells that plays an essential role in many processes. PP2A mutations in Schizosaccharomyces pombe result in defects of cell cycle control, cytokinesis and morphogenesis. Which PP2A substrates are responsible for these changes is not known. In this work, we searched for PP2A substrates in S. pombe using two approaches, 2D-DIGE analysis of PP2A complex mutants and identification of PP2A interacting proteins. In both cases, we used MS to identify proteins of interest. In the DIGE experiment, we compared proteomes of wild-type S. pombe, deletion of pta2, the phosphoactivator of the PP2A catalytic subunit, and pab1-4, a mutant of B-type PP2A regulatory subunit. A total of 1742 protein spots were reproducibly resolved by 2D-DIGE and 51 spots demonstrated significant changes between PP2A mutants and the wild-type control. MS analysis of these spots identified 27 proteins that include key regulators of glycerol synthesis, carbon metabolism, amino acid biosyntesis, vitamin production, and protein folding. Importantly, we independently identified a subset of these proteins as PP2A binding partners by affinity precipitation, suggesting they may be direct targets of PP2A. We have validated our approach by demonstrating that phosphorylation of Gpd1, a key enzyme in glycerol biogenesis, is regulated by PP2A and that ability of cells to respond to osmotic stress by synthesizing glycerol is compromised in the PP2A mutants. Our work contributes to a better understanding of PP2A function and identifies potential PP2A substrates.