title: Monoubiquitin carries a novel internalization signal that is appended to activated receptors.
abstract: Ubiquitin modification of signal transducing receptors at the plasma membrane is necessary for rapid receptor internalization and downregulation. We have investigated whether ubiquitylation alters a receptor cytoplasmic tail to reveal a previously masked internalization signal, or whether ubiquitin itself carries an internalization signal. Using an alpha-factor receptor-ubiquitin chimeric protein, we demonstrate that monoubiquitin can mediate internalization of an activated receptor that lacks all cytoplasmic tail sequences. Furthermore, fusion of ubiquitin in-frame to the stable plasma membrane protein Pma1p stimulates endocytosis of this protein. Ubiquitin does not carry a functional tyrosine- or di-leucine-based internalization signal. Instead, the three-dimensional structure of the folded ubiquitin polypeptide carries an internalization signal that consists of two surface patches surrounding the critical residues Phe4 and Ile44. We conclude that ubiquitin functions as a novel regulated internalization signal that can be appended to a plasma membrane protein to trigger downregulation.
authors.affiliation: Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, Evanston, IL 60208, USA.
authors.forename: S C
authors.initials: SC
authors.lastname: Shih
authors.forename: K E
authors.initials: KE
authors.lastname: Sloper-Mould
authors.forename: L
authors.initials: L
authors.lastname: Hicke
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D002462:Cell Membrane / Q000378:metabolism; D007527:Isoenzymes / Q000737:chemistry / Q000378:metabolism; D007532:Isoleucine; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010649:Phenylalanine; D011487:Protein Conformation; D006180:Proton-Translocating ATPases / Q000737:chemistry / Q000378:metabolism; D051964:Receptors, Mating Factor; D018000:Receptors, Peptide / Q000737:chemistry* / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D017384:Sequence Deletion; D015398:Signal Transduction; D014157:Transcription Factors; D014452:Ubiquitins / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D007527:Isoenzymes; C434364:PMA2 protein, S cerevisiae; D051964:Receptors, Mating Factor; D018000:Receptors, Peptide; D011993:Recombinant Fusion Proteins; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors; D014452:Ubiquitins; D007532:Isoleucine; D010649:Phenylalanine; C092477:PMA1 protein, S cerevisiae; D006180:Proton-Translocating ATPases