Monoubiquitin carries a novel internalization signal that is appended to activated receptors

Ubiquitin modification of signal transducing receptors at the plasma membra ne is necessary for rapid receptor internalization and downregulation, We h ave 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-ub iquitin chimeric protein, we demonstrate that monoubiquitin can mediate int ernalization of an activated receptor that lacks all cytoplasmic tail seque nces. Furthermore, fusion of ubiquitin in-frame to the stable plasma membra ne protein Pma1p stimulates endocytosis of this protein. Ubiquitin does not carry a functional tyrosine- or di-leucine-based internalization signal. I nstead, the three-dimensional structure of the folded ubiquitin polypeptide carries an internalization signal that consists of two surface patches sur rounding the critical residues Phe4 and Ile44. We conclude that ubiquitin f unctions as a novel regulated internalization signal that can be appended t o a plasma membrane protein to trigger downregulation.