Ubiquitination of a Yeast Plasma Membrane Receptor Signals Its Ligand-Stimulated Endocytosis

Binding of α factor to Ste2p, a G protein–coupled plasma membrane receptor, activates a signal transduction pathway and stimulates endocytosis of the receptor–ligand complex. Ligand binding also induces ubiquitination of the Ste2p cytoplasmic tail. Protein ubiquitination is required for stimulated endocytosis of Ste2p, as internalization is 5- to 15-fold slower in ubc mutants that lack multiple ubiquitin-conjugating enzymes. In a C-terminal truncated form of Ste2p that is rapidly ubiquitinated and endocytosed in response to ligand binding, a single lysine to arginine substitution in its cytoplasmic tail eliminates both ubiquitination and internalization. Thus, ubiquitination of Ste2p itself is required for ligand-stimulated endocytosis. We propose that ubiquitination mediates degradation of receptor–ligand complexes, not via the proteasome, but by acting as a signal for endocytosis leading to subsequent degradation in the lysosome/vacuole.