title: Elimination of defective alpha-factor pheromone receptors.
abstract: This report compares trafficking routes of a plasma membrane protein that was misfolded either during its synthesis or after it had reached the cell surface. A temperature-sensitive mutant form of the yeast alpha-factor pheromone receptor (ste2-3) was found to provide a model substrate for quality control of plasma membrane proteins. We show for the first time that a misfolded membrane protein is recognized at the cell surface and rapidly removed. When the ste2-3 mutant cells were cultured continuously at 34 degrees C, the mutant receptor protein (Ste2-3p) failed to accumulate at the plasma membrane and was degraded with a half-life of 4 min, compared with a half-life of 33 min for wild-type receptor protein (Ste2p). Degradation of both Ste2-3p and Ste2p required the vacuolar proteolytic activities controlled by the PEP4 gene. At 34 degrees C, Ste2-3p comigrated with glycosylated Ste2p on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, indicating that Ste2-3p enters the secretory pathway. Degradation of Ste2-3p did not require delivery to the plasma membrane as the sec1 mutation failed to block rapid turnover. Truncation of the C-terminal cytoplasmic domain of the mutant receptors did not permit accumulation at the plasma membrane; thus, the endocytic signals contained in this domain are unnecessary for intracellular retention. In the pep4 mutant, Ste2-3p accumulated as series of high-molecular-weight species, suggesting a potential role for ubiquitin in the elimination process. When ste2-3 mutant cells were cultured continuously at 22 degrees C, Ste2-3p accumulated in the plasma membrane. When the 22 degrees C culture was shifted to 34 degrees C, Ste2-3p was removed from the plasma membrane and degraded by a PEP4-dependent mechanism with a 24-min half-life; the wild-type Ste2p displayed a 72-min half-life. Thus, structural defects in Ste2-3p synthesized at 34 degrees C are recognized in transit to the plasma membrane, leading to rapid degradation, and Ste2-3p that is preassembled at the plasma membrane is also removed and degraded following a shift to 34 degrees C.
authors.affiliation: Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Worcester 01655-0122, USA. jennessd@ummed.edu
authors.forename: D D
authors.initials: DD
authors.lastname: Jenness
authors.forename: Y
authors.initials: Y
authors.lastname: Li
authors.forename: C
authors.initials: C
authors.lastname: Tipper
authors.forename: P
authors.initials: P
authors.lastname: Spatrick
mesh_terms: D001692:Biological Transport; D002451:Cell Compartmentation; D002462:Cell Membrane / Q000378:metabolism; D003001:Cloning, Molecular; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D009154:Mutation; D011487:Protein Conformation; D017510:Protein Folding; D051964:Receptors, Mating Factor; D018000:Receptors, Peptide / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012098:Reproduction; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D017422:Sequence Analysis, DNA; D014157:Transcription Factors; D014617:Vacuoles / Q000378:metabolism
chemical_list: D005656:Fungal Proteins; D051964:Receptors, Mating Factor; D018000:Receptors, Peptide; D014157:Transcription Factors