Distinct monoubiquitin signals in receptor endocytosis

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Abstract

Numerous cellular proteins are post-translationally modified by the addition of the small modifier protein ubiquitin (Ub). The functional consequences of the type of ubiquitination vary, such that polyubiquitinated proteins are targeted for degradation by the proteasome, whereas monoubiquitination is implicated in other cellular functions, including endocytic trafficking and DNA repair. The monoubiquitination of trafficking cargoes, such as receptors and associated proteins, as well as of endocytic accessory Ub-binding proteins, raises the question of the precise function of monoubiquitin signals in the endocytic route. Recent biochemical and genetic evidence shows that multiple monoubiquitination of epidermal growth factor and platelet-derived growth factor receptors provides trafficking and sorting tags that ensure receptor endocytosis and degradation, whereas monoubiquitination of accessory proteins might play a role in regulating their function as Ub-receptors in the endosome.

Section snippets

Ubiquitin in mono, multi- and poly-modifications

Ubiquitin is a highly conserved 76 amino acid polypeptide that is covalently attached to target proteins via an isopeptide bond between the C-terminal glycine of ubiquitin and the ε-amino group of a lysine in substrate proteins. This occurs through a three-step process involving ubiquitin-activating (E1), ubiquitin-conjugating (E2) and ubiquitin ligase (E3) enzymes [5]. The addition of a single ubiquitin to a substrate is defined as monoubiquitination [6] (Figure 1). Moreover, several lysine

Monoubiquitination in receptor internalization and endocytosis

Ligand-induced ubiquitination of several plasma membrane receptors, including numerous RTKs, has been implicated in the regulation of their internalization and endocytosis 18, 19, 20, 21, 22, 23, 24, 25, 26 (Figure 2). The first clue that monoubiquitin, and not ubiquitin chains, controls these processes came from studies on Ste2p, a pheromone G protein-coupled receptor in Saccharomyces cerevisiae [24]. Ste2p removal from the cell surface and delivery for degradation to the yeast vacuole, an

Role of Ub-receptors in the endocytic pathways

The identification of ubiquitin-binding proteins (Ub-receptors), containing ubiquitin-interacting motifs (UIMs) 37, 38, ubiquitin-associated (UBA) [39], ubiquitin-conjugating enzyme-like (UBC)/ubiquitin E2 variant (UEV) [40] or Cue1-homologous (CUE) domains 41, 42, has revealed how trafficking of ubiquitinated RTKs might be governed from the cell surface all the way to the lysosome (Figure 2, see also [13]). Ub-receptors are compartmentalized along the endocytic pathway and might function as

Why is multiple monoubiquitination needed for receptor endocytosis?

Recent studies demonstrated that multiple monoubiquitination, but not formation of ubiquitin chains, is associated with ligand-induced ubiquitination of EGF and PDGF receptors 3, 4. The smeary sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) pattern of activated RTKs is due to receptor phosphorylation, glycosylation and their monoubiquitination at multiple sites [3]. These findings have provided a molecular explanation of why activated RTKs, which were long thought to be

Monoubiquitination of Ub-receptors

Several Ub-receptors are endowed with the dual function of binding ubiquitin and being monoubiquitinated upon growth factor stimulation [13]. Monoubiquitination of several Ub-receptors, including Eps15, epsins and Hrs, is directed by the HECT (homologous to E6-associated protein C terminus)-type ubiquitin ligase Nedd4, which is not directly associated with activated RTKs 38, 63, thus representing a pathway that is distinct from Cbl-mediated multiple monoubiquitination of RTKs and associated

Concluding remarks

It is becoming increasingly evident that monoubiquitin signals have different roles in controlling RTK downregulation. Multiple monoubiquitin molecules attached to a trafficking cargo (i.e. activated receptors and associated proteins) might provide a targeting signal that ensures efficient endocytic sorting and lysosomal degradation, whereas monoubiquitination of Ub-receptors might affect their ability to act as gating receptors in the endosome. In spite of the rapid recent progress in

Acknowledgements

We thank Carl-Henrik Heldin, Werner Muller-Esterl, Wallace Langdon and members of the Dikic laboratory for critical comments on this article. The authors' work is supported by grants from AIRC (Italian Association for Cancer Research), Human Science Frontier Program, IARC (International Association for Cancer Research), The European Community (VI Framework), the Telethon Foundation and the Italian Ministry of Health (P.P.D.F.) and by grants from the Boehringer Ingelheim Foundation and Swedish

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