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Originally published as Genetics Published Articles Ahead of Print on June 11, 2007.

Genetics, Vol. 176, 2139-2150, August 2007, Copyright © 2007
doi:10.1534/genetics.107.072835

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Efficient Tor Signaling Requires a Functional Class C Vps Protein Complex in Saccharomyces cerevisiae

Sara A. Zurita-Martinez*,1, Rekha Puria*,1, Xuewen Pan{dagger}, Jef D. Boeke{ddagger} and Maria E. Cardenas*,2

* Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, North Carolina 27710, {dagger} Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030 and {ddagger} Department of Molecular Biology and Genetics, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

2 Corresponding author: Department of Molecular Genetics and Microbiology, Duke University Medical Center, 322 CARL Bldg., Box 3546, Research Dr., Durham, NC 27710.
E-mail: carde004{at}mc.duke.edu

The Tor kinases regulate responses to nutrients and control cell growth. Unlike most organisms that only contain one Tor protein, Saccharomyces cerevisiae expresses two, Tor1 and Tor2, which are thought to share all of the rapamycin-sensitive functions attributable to Tor signaling. Here we conducted a genetic screen that defined the global TOR1 synthetic fitness or lethal interaction gene network. This screen identified mutations in distinctive functional categories that impaired vacuolar function, including components of the EGO/Gse and PAS complexes that reduce fitness. In addition, tor1 is lethal in combination with mutations in class C Vps complex components. We find that Tor1 does not regulate the known function of the class C Vps complex in protein sorting. Instead class C vps mutants fail to recover from rapamycin-induced growth arrest or to survive nitrogen starvation and have low levels of amino acids. Remarkably, addition of glutamate or glutamine restores viability to a tor1 pep3 mutant strain. We conclude that Tor1 is more effective than Tor2 at providing rapamycin-sensitive Tor signaling under conditions of amino acid limitation, and that an intact class C Vps complex is required to mediate intracellular amino acid homeostasis for efficient Tor signaling.




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