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Mol Cell Biol, February 1998, p. 779-789, Vol. 18, No. 2
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Role for the Ubiquitin-Proteasome System in the
Vacuolar Degradation of Ste6p, the a-Factor Transporter
in Saccharomyces cerevisiae
Diego
Loayza and
Susan
Michaelis*
Department of Cell Biology and Anatomy, The
Johns Hopkins University School of Medicine, Baltimore, Maryland
21205
Received 22 August 1997/Returned for modification 6 October
1997/Accepted 29 October 1997
Ste6p, the a-factor transporter in Saccharomyces
cerevisiae, is a multispanning membrane protein with 12 transmembrane spans and two cytosolic ATP binding domains. Ste6p
belongs to the ATP binding cassette (ABC) superfamily and provides an
excellent model for examining the intracellular trafficking of a
complex polytopic membrane protein in yeast. Previous studies have
shown that Ste6p undergoes constitutive endocytosis from the plasma
membrane, followed by delivery to the vacuole, where it is degraded in
a Pep4p-dependent manner, even though only a small portion of
Ste6p is exposed to the vacuolar lumen where the
Pep4p-dependent proteases reside. Ste6p is known to be
ubiquitinated, a modification that may facilitate its endocytosis. In
the present study, we further investigated the intracellular
trafficking of Ste6p, focusing on the role of the
ubiquitin-proteasome machinery in the metabolic degradation of
Ste6p. We demonstrate by pulse-chase analysis that the
degradation of Ste6p is impaired in mutants that exhibit defects in the
activity of the proteasome (doa4 and
pre1,2). Likewise, by immunofluorescence, we
observe that Ste6p accumulates in the vacuole in the
doa4 mutant, as it does in the vacuolar
protease-deficient pep4 mutant. One model consistent with
our results is that the degradation of Ste6p, the bulk of which is
exposed to the cytosol, requires the activity of both the cytosolic
proteasomal degradative machinery and the vacuolar lumenal proteases,
acting in a synergistic fashion. Alternatively, we discuss a
second model whereby the ubiquitin-proteasome system may indirectly
influence the Pep4p-dependent vacuolar degradation of Ste6p. This study
establishes that Ste6p is distinctive in that two independent
degradative systems (the vacuolar Pep4p-dependent proteases and the
cytosolic proteasome) are both involved, either directly or indirectly,
in the metabolic degradation of a single substrate.
*
Corresponding author. Mailing address: Department of
Cell Biology and Anatomy, The Johns Hopkins University School of
Medicine, 725 North Wolfe St., Baltimore, MD 21205. Phone: (410)
955-8286. Fax: (410) 955-4129. E-mail:
susan_michaelis{at}qmail.bs.jhu.edu.
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