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Roles for PI(3,5)P2 in nutrient sensing through TORC1.

Natsuko Jin1, Kai Mao, Yui Jin

  • 1Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109 Department of Molecular, Cellular, and Developmental Biology and Department of Biological Chemistry, University of Michigan, Ann Arbor, MI 48109 Department of Internal Medicine and Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI 48109 Department of Molecular Biology, University of Geneva, Geneva 1211, Switzerland Department of Cell and Developmental Biology, University of Michigan Medical School, Ann Arbor, MI 48109.

Molecular Biology of the Cell
|January 31, 2014
PubMed
Summary

This study explores the role of a lipid called PI(3,5)P2 in regulating a protein complex called TORC1, which controls cell growth in response to nutrients. The researchers found that PI(3,5)P2 is essential for TORC1 activity in yeast and that it functions on the vacuole, which is the yeast equivalent of the lysosome. They showed that PI(3,5)P2 helps recruit TORC1 substrates like Sch9 to the vacuole, where they are phosphorylated. The study also found that PI(3,5)P2 is necessary for TORC1 to regulate multiple downstream processes, including autophagy and endocytosis. These findings suggest that PI(3,5)P2 serves as a general regulator of TORC1 and provides a platform for signaling from lysosomes.

Keywords:
TORC1 regulationlysosome signalingautophagy inhibitionendocytosis regulation

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Area of Science:

  • Cell signaling in metabolic regulation
  • Membrane lipid function in TORC1 activation
  • Autophagy and nutrient sensing mechanisms

Background:

TORC1 is a key protein kinase that responds to nutrient availability to control cell growth. It is known that TORC1 must localize to lysosomes in mammals for activation. PI(3,5)P2, a lipid found in endosomes, has been linked to TORC1 activity in specific cell types like adipocytes. However, it was unclear whether PI(3,5)P2 plays a general role in TORC1 regulation. The exact subcellular site where PI(3,5)P2 influences TORC1 was also unknown. Prior research showed that insulin can stimulate TORC1 through PI(3,5)P2, but this did not clarify its broader function. No prior work had resolved whether PI(3,5)P2 is essential for TORC1 activity in other organisms or cellular contexts. This gap motivated an investigation into the role of PI(3,5)P2 in TORC1 regulation across species. The uncertainty about its mechanism and location led to a study in yeast to explore these questions. This study aimed to determine whether PI(3,5)P2 is a general regulator of TORC1 and where it acts.

Purpose Of The Study:

The study aimed to determine whether PI(3,5)P2 is a general regulator of TORC1 activity across species and to identify the subcellular location where it functions. Researchers wanted to test if PI(3,5)P2 is essential for TORC1 in yeast, which lacks adipocytes and insulin signaling. They also sought to clarify whether PI(3,5)P2 acts on the vacuole, the yeast equivalent of the lysosome. The motivation was to expand the understanding of TORC1 regulation beyond mammalian models. The researchers were particularly interested in how PI(3,5)P2 might influence TORC1 substrates. They also wanted to investigate if PI(3,5)P2 contributes to downstream signaling pathways through TORC1. The study aimed to test whether PI(3,5)P2 is necessary for TORC1 to phosphorylate multiple targets. This would help determine if PI(3,5)P2 serves as a general platform for TORC1 signaling.

Main Methods:

The researchers used yeast as a model system to study TORC1 regulation by PI(3,5)P2. They examined TORC1 activity in cells lacking the ability to produce PI(3,5)P2. They also tested whether TORC1 substrates are recruited to the vacuole in a PI(3,5)P2-dependent manner. To assess this, they used genetic and biochemical approaches, including mutant strains and protein interaction assays. They monitored the phosphorylation status of TORC1 substrates like Sch9, Atg13, and Npr1. The team used fluorescence microscopy to determine the subcellular localization of these proteins. They also performed functional assays to evaluate the effects of PI(3,5)P2 on autophagy and endocytosis. These methods allowed them to determine the role of PI(3,5)P2 in TORC1 signaling and downstream processes.

Main Results:

The study found that PI(3,5)P2 is required for TORC1 activity in yeast, suggesting a conserved role. The researchers observed that TORC1 substrates, including Sch9, are recruited to the vacuole by binding to PI(3,5)P2. This recruitment is necessary for phosphorylation by TORC1. They also found that PI(3,5)P2 is essential for the phosphorylation of Atg13, which inhibits autophagy. Npr1 phosphorylation by TORC1 was also dependent on PI(3,5)P2. The absence of PI(3,5)P2 led to reduced phosphorylation of these substrates. The study showed that PI(3,5)P2 supports multiple downstream pathways via TORC1. These findings suggest that PI(3,5)P2 provides a platform for TORC1 signaling from the vacuole.

Conclusions:

The authors conclude that PI(3,5)P2 is a general regulator of TORC1 activity, not limited to mammalian cells. They suggest that PI(3,5)P2 functions on the vacuole, the yeast equivalent of the lysosome. The study shows that PI(3,5)P2 is required for TORC1 to phosphorylate multiple substrates, including Sch9, Atg13, and Npr1. The researchers propose that PI(3,5)P2 provides a platform for TORC1 signaling from lysosomes. They note that this lipid supports multiple downstream pathways, such as autophagy and endocytosis. The findings suggest that PI(3,5)P2 plays a conserved role in TORC1 regulation. The study does not claim that PI(3,5)P2 is the only regulator of TORC1. The authors do not propose future directions or new drug targets.

The study found that PI(3,5)P2 is required for TORC1 activity in yeast and supports phosphorylation of multiple substrates.

Sch9 is a TORC1 substrate that is recruited to the vacuole by direct interaction with PI(3,5)P2.

The vacuole is the site where PI(3,5)P2 regulates TORC1, suggesting it functions similarly to lysosomes in mammals.

Atg13 is phosphorylated by TORC1 in a PI(3,5)P2-dependent manner, which inhibits autophagy.

Npr1 is phosphorylated by TORC1 on the vacuole, and this modification releases its inhibitory function.

The authors suggest that PI(3,5)P2 provides a platform for TORC1 signaling from lysosomes.