The TOR pathway modulates cytoophidium formation in Schizosaccharomyces pombe

Christos Andreadis,Lydia Hulme,Katherine Wensley,Ji-Long Liu

doi:10.1074/jbc.ra119.009913

Christos Andreadis, Lydia Hulme + Show 2 more

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https://doi.org/10.1074/jbc.ra119.009913

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Abstract

CTP synthase (CTPS) has been demonstrated to form evolutionarily-conserved filamentous structures termed cytoophidia whose exact cellular functions remain unclear, but they may play a role in intracellular compartmentalization. We have previously shown that the mammalian target of rapamycin complex 1 (mTORC1)–S6K1 pathway mediates cytoophidium assembly in mammalian cells. Here, using the fission yeast Schizosaccharomyces pombe as a model of a unicellular eukaryote, we demonstrate that the target of rapamycin (TOR)-signaling pathway regulates cytoophidium formation (from the S. pombe CTPS ortholog Cts1) also in S. pombe. Conducting a systematic analysis of all viable single TOR subunit–knockout mutants and of several major downstream effector proteins, we found that Cts1 cytoophidia are significantly shortened and often dissociate when TOR is defective. We also found that the activities of the downstream effector kinases of the TORC1 pathway, Sck1, Sck2, and Psk1 S6, as well as of the S6K/AGC kinase Gad8, the major downstream effector kinase of the TORC2 pathway, are necessary for proper cytoophidium filament formation. Interestingly, we observed that the Crf1 transcriptional corepressor for ribosomal genes is a strong effector of Cts1 filamentation. Our findings connect TOR signaling, a major pathway required for cell growth, with the compartmentalization of the essential nucleotide synthesis enzyme CTPS, and we uncover differences in the regulation of its filamentation among higher multicellular and unicellular eukaryotic systems.

Highlights

CTP synthase (CTPS) has been demonstrated to form evolutionarily-conserved filamentous structures termed cytoophidia whose exact cellular functions remain unclear, but they may play a role in intracellular compartmentalization
As in previous studies of cytoophidia in yeasts [17, 19, 23, 54, 55], endogenous tagging was preferred over immunostaining because the use of the latter was hindered by the enzymatic treatment necessary to render the fission yeast cell wall permeable to antibodies, a procedure that leads to dissociation of the sensitive structure of cytoophidia (Fig. S1)
In agreement with our data, it has been demonstrated that rapamycin only mildly reduces the growth rate in fission yeast cells, whereas TORC1 activation is significantly inhibited (56 –58), as is in mammalian cells treated with rapamycin [51]

Summary

Introduction

CTP synthase (CTPS) has been demonstrated to form evolutionarily-conserved filamentous structures termed cytoophidia whose exact cellular functions remain unclear, but they may play a role in intracellular compartmentalization. Using the fission yeast Schizosaccharomyces pombe as a model of a unicellular eukaryote, we demonstrate that the target of rapamycin (TOR)-signaling pathway regulates cytoophidium formation (from the S. pombe CTPS ortholog Cts1) in S. pombe. Our findings connect TOR signaling, a major pathway required for cell growth, with the compartmentalization of the essential nucleotide synthesis enzyme CTPS, and we uncover differences in the regulation of its filamentation among higher multicellular and unicellular eukaryotic systems. It has been shown that CTPS filaments may contain an inactive or active form of the enzyme in different organisms [29]. The formation and regulation of these filaments remain elusive (15, 16, 18, 20, 30 –32)

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