Phosphorylation of Wat1, human Lst8 homolog is critical for the regulation of TORC2 –Gad8 dependent pathway in fission yeast Schizosacchromyces pombe

TORC1 mediated regulation of mitochondrial integrity and calcium ion homeostasis by Wat1/mLst8 in S. pombe

The mTOR complexes play a fundamental role in mitochondrial biogenesis and cellular homeostasis. Wat1, an ortholog of mammalian Lst8 is an important component of TOR complex and is essential for the regulation of downstream signaling. Earlier we reported the role of Wat1 in oxidative stress response. Here, we have shown that the abrogation of wat1 causes respiratory defects and mitochondrial depolarization that leads to a decrease in ATP production. The confocal and electron microscopy in wat1Δ cells revealed the fragmented mitochondrial morphology implying its role in mitochondrial fission. Furthermore, we also showed its role in autophagy and the maintenance of calcium ion homeostasis. Additionally, tor2-287 mutant cells also exhibit defects in mitochondrial integrity indicating the TORC1-dependent involvement of Wat1 in the maintenance of mitochondrial homeostasis. The interaction studies of Wat1 and Tor2 with Por1 and Mmm1 proteins revealed a plausible cross-talk between mitochondria and endoplasmic reticulum through the Mitochondria-associated membranes (MAM) and endoplasmic reticulum-mitochondria encounter structure (ERMES) complex, involving TORC1. Taken together, this study demonstrates the involvement of Wat1/mLst8 in harmonizing various mitochondrial functions, redox status, and Ca2+ homeostasis.

Critical role of Wat1/Pop3 in regulating the TORC1 signalling pathway in fission yeast S. pombe

The target of rapamycin (TOR), a major pathway for the regulation of cell growth and proliferation is conserved from yeast to humans. Fission yeast contains two tor complexes, TORC1 is crucial for cell growth while TORC2 gets activated under stress conditions. Pop3/Wat1, a mammalian Lst8 ortholog is an important component of both TOR complexes and has been implicated in the oxidative stress response pathway. Here in this study, the genetic interaction analysis revealed a synthetic lethal interaction of wat1 with tor2-287 mutant cells. Co-immunoprecipitation analysis revealed Wat1 interacts with TORC1 components Tor2, Mip1, and Tco89 while wat1-17 mutant protein fails to interact with these proteins. In the absence of Wat1, the cells arrest at G1 phase with reduced cell size at non-permissive temperature reminiscent of tor2-287 mutant phenotype. Similarly, inactivation of Wat1 results in the failure of TORC1 mediated phosphorylation of Psk1 and Rps602, leading to dysregulation of amino acid permeases and delocalization of Gaf1, a DNA binding transcription factor. Overall, we have hypothesized that Wat1/Pop3 is required to execute the function of TORC1.

Pyrogallol induces oxidative stress defects in the fission yeast S. pombe

Apart from the beneficial roles of pyrogallol in industries, it also tends to produce free radicals that trigger apoptosis in human cells. In this study, we checked the toxic effect of pyrogallol in fission yeast S. pombe cells. We observed that the wild type and wat1/pop3 delete cells were unable to grow on plates containing pyrogallol in a dose-dependent manner. Furthermore, the wat1/pop3 delete cells exhibit higher sensitivity against pyrogallol as compared to wild type cells suggesting that the pyrogallol induces oxidative stress. The exposure to pyrogallol also leads to the production of ROS and affects the sporulation in S. pombe.

The N-terminus region of Drp1, a Rint1 family protein is essential for cell survival and its interaction with Rad50 protein in fission yeast S.pombe

BACKGROUND

Defects in DNA repair pathway can lead to double-strand breaks leading to genomic instability. Earlier we have shown that S.pombe Drp1, a Rint1/Tip1 family protein is required for the recovery from DNA damage.

METHODS

Various truncations of Drp1 protein were constructed and their role in DNA damage response and interaction with Rad50 protein has been studied by co-immunoprecipitation and pull-down assays.

RESULTS

The structural and functional analysis of Drp1 protein revealed that the N-terminus region of Drp1 is indispensable for the survival. The C-terminus truncation mutants, drp1C1Δ and drp1C2Δ exhibit temperature sensitive phenotype and are hypersensitive against DNA damaging agents with elevated level of Rad52-YFP foci at non-permissive temperature indicating the impairment for DNA damage repair pathway. The essential N-terminus region of Drp1 interacts with the C-terminus region of Rad50 and might be involved in influencing the MRN/X function. Small-angle X-ray (SAXS) analysis revealed three-domain like shapes in Drp1 protein while the C-terminus region of Rad50 exhibit unusual bulges. Computational docking studies revealed the amino acid residues at the C-terminus region of Rad50 that are involved in the interaction with the residues present at the N-terminal region of Drp1 indicating the importance of the N-terminal region of Drp1 protein.

CONCLUSIONS

We have identified the region of Drp1 and Rad50 proteins that are involved in the interaction and their role in the DNA damage response pathway has been analyzed.

GENERAL SIGNIFICANCE

The functional and structural aspects of fission yeast Drp1 protein and its interaction with Rad50 have been elucidated.