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

Abstract

Binding of alpha 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.

Regulatory mechanisms that modulate signalling by G-protein-coupled receptors

“…The sequence SINN-DAKSS within the C-terminal tail of Ste2p is necessary and sufficient for receptor endocytosis [225]. Ligand binding induces ubiquitination of the Ste2p C-terminal tail, which is required for endocytosis, since internalization is 5-15-fold slower in yeast mutants that lack multiple ubiquitin-conjugating enzymes [222]. Mutation of the lysine residue in the SINNDAKSS motif abolishes ubiquitination, supporting the proposal that ubiquitination is an endocytosis signal for Ste2p.…”

“…For example, the T cell antigen receptor, the c-kit receptor and the platelet-derived growth factor β receptor undergo ligand-mediated ubiquitination within the C-terminal tail and subsequent degradation [217][218][219][220][221]. Ste2p, a GPCR in yeast for α pheromone, also undergoes ubiquitination [222]. Agonist binding induces internalization of Ste2p by a clathrin-dependent mechanism, and the internalized receptor is transported to the vacuole (the yeast equivalent of a lysosome), where it is degraded [223,224].…”

A stimulus needed for the study of membrane traffic in hepatocytes

“…It is now known that the attachment of a ubiquitin moiety to proteins plays a major role in lysosomal targeting and degradation and is required for the passage of certain cargo molecules into and out of vesicles of the endocytic pathway as well as signaling processes. [11][12][13][14][15][16] Ubiquitin-mediated down-regulation of receptor tyrosine kinases has been observed for a number of different receptors, with the best studied example being EGFR. Importantly, this ubiquitin-dependent sorting to the lysosome is mediated in part by a series of multiprotein complexes termed endosomal sorting complexes required for sorting (ESCRTs).…”

Fine regulation of Saccharomyces cerevisiae MAPK pathways by post‐translational modifications

“…In the field of yeast MAPK signal transduction, ubiquitination was first observed for the mating pathway, in which it was shown to contribute to attenuation of MAPK activation (see Figure 1). Ubiquitination is required for endocytosis of the receptors operating in the pathway, leading to their subsequent degradation in the lysosome/vacuole [30,51]. In this pathway, the MAPKK Ste11 also appears to be regulated by ubiquitination [13].…”