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Published 28 April 2003. doi:10.1083/jcb.200210141
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© The Rockefeller University Press, 0021-9525/2003/4/333 $5.00
The Journal of Cell Biology, Volume 161, Number 2, 333-347

Article

 LST8 negatively regulates amino acid biosynthesis as a component of the TOR pathway

Esther J. Chen and Chris A. Kaiser

Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139

Address correspondence to Chris A. Kaiser 68-533, Dept. of Biology, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139. Tel.: (617) 253-9804. Fax: (617) 253-6622. E-mail: ckaiser{at}mit.edu

LST8, a Saccharomyces cerevisiae gene encoding a 34-kD WD-repeat protein, was identified by mutations that caused defects in sorting Gap1p to the plasma membrane. Here, we report that the Gap1p sorting defect in the lst8-1 mutant results from derepression of Rtg1/3p activity and the subsequent accumulation of high levels of intracellular amino acids, which signal Gap1p sorting to the vacuole. To identify the essential function of Lst8p, we isolated lst8 mutants that are temperature-sensitive for growth. These mutants show hypersensitivity to rapamycin and derepressed Gln3p activity like cells with compromised TOR pathway activity. Like tor2 mutants, lst8 mutants also have cell wall integrity defects. Confirming a role for Lst8p in the TOR pathway, we find that Lst8p associates with both Tor1p and Tor2p and is a peripheral membrane protein that localizes to endosomal or Golgi membranes and cofractionates with Tor1p. Further, we show that a sublethal concentration of rapamycin mimics the Gap1p sorting defect of an lst8 mutant. Finally, the different effects of lst8 alleles on the activation of either the Rtg1/3p or Gln3p transcription factors reveal that these two pathways constitute distinct, genetically separable outputs of the Tor–Lst8 regulatory complex.

Key Words: GAP1; rapamycin; GLN3; Golgi; RTG1

* Abbreviation used in this paper: YPD, yeast extract/peptone/dextrose.


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