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Published online 16 June 2003. doi:10.1083/jcb.200303088
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© The Rockefeller University Press, 0021-9525/2003/6/1117 $5.00
The Journal of Cell Biology, Volume 161, Number 6, 1117-1131

Article

Ergosterol is required for targeting of tryptophan permease to the yeast plasma membrane



Kyohei Umebayashi and Akihiko Nakano

Molecular Membrane Biology Laboratory, RIKEN, Saitama 351-0198, Japan

Address correspondence to Akihiko Nakano, Molecular Membrane Biology Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. Tel.: 81-48-467-9547. Fax: 81-48-462-4679. E-mail: nakano{at}postman.riken.go.jp

It was known that the uptake of tryptophan is reduced in the yeast erg6 mutant, which is defective in a late step of ergosterol biosynthesis. Here, we show that this is because the high affinity tryptophan permease Tat2p is not targeted to the plasma membrane. In wild-type cells, the plasma membrane localization of Tat2p is regulated by the external tryptophan concentration. Tat2p is transported from the Golgi apparatus to the vacuole at high tryptophan, and to the plasma membrane at low tryptophan. However, in the erg6 mutant, Tat2p is missorted to the vacuole at low tryptophan. The plasma membrane targeting of Tat2p is dependent on detergent-insoluble membrane domains, suggesting that sterol affects the sorting through the organization of lipid rafts. The erg6 mutation also caused missorting to the multivesicular body pathway in late endosomes. Thus, sterol composition is crucial for protein sorting late in the secretory pathway. Tat2p is subject to polyubiquitination, which acts as a vacuolar-targeting signal, and the inhibition of this process suppresses the Tat2p sorting defects of the erg6 mutant. The sorting mechanisms of Tat2p that depend on both sterol and ubiquitin will be discussed.

Key Words: ERG6; Tat2p; raft; multivesicular body; ubiquitin

The online version of this article includes supplemental material.

K. Umebayashi's present address is Department of Cell Genetics, National Institute of Genetics, 1111 Yata, Mishima, Shizuoka 411-8540, Japan.

* Abbreviations used in this paper: MVB, multivesicular body; MVL, mevalonic acid lactone; VPS, vacuolar protein sorting.


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