Monoubiquitin carries a novel internalization signal that is appended to activated receptors.
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.
Shih
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SC
Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, Evanston, IL 60208, USA.
Sloper-Mould
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Hicke
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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*
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