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J. Cell Biol.,
Volume 141, Number 2, April 20, 1998 349-358
-Factor Receptor
Is Required for Its Ubiquitination and Internalization


* Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Evanston, Illinois 60208; and G protein-coupled (GPC) receptors are
phosphorylated in response to ligand binding, a modification that promotes receptor desensitization or downregulation. The
Department of Biochemistry, Biozentrum, University of Basel, Basel, Switzerland CH-4056
-factor pheromone receptor (Ste2p)
of Saccharomyces cerevisiae is a GPC receptor that is hyperphosphorylated and ubiquitinated upon binding
-factor. Ubiquitination triggers Ste2p internalization
into the endocytic pathway. Here we demonstrate that
phosphorylation of Ste2p promotes downregulation by
positively regulating ubiquitination and internalization.
Serines and a lysine are essential elements of the Ste2p
SINNDAKSS internalization signal that can mediate
both constitutive and ligand-stimulated endocytosis.
The SINNDAKSS serines are required for receptor
phosphorylation which, in turn, facilitates ubiquitination of the neighboring lysine. Constitutive phosphorylation is required to promote constitutive internalization, and is also a prerequisite for ligand-induced phosphorylation at or near the SINNDAKSS sequence.
Mutants defective in yeast casein kinase I homologues
are unable to internalize
-factor, and do not phosphorylate or ubiquitinate the receptor, indicating that these
kinases play a direct or indirect role in phosphorylating
the receptor. Finally, we provide evidence that the primary function of phosphorylation controlled by the
SINNDAKSS sequence is to trigger receptor internalization, demonstrating that phosphorylation-dependent
endocytosis is an important mechanism for the downregulation of GPC receptor activity.
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