Reference: Mattera R, et al. (2006) The Rab5 guanine nucleotide exchange factor Rabex-5 binds ubiquitin (Ub) and functions as a Ub ligase through an atypical Ub-interacting motif and a zinc finger domain. J Biol Chem 281(10):6874-83

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Abstract

Rabex-5, the mammalian orthologue of yeast Vps9p, is a guanine nucleotide exchange factor for Rab5. Rabex-5 forms a tight complex with Rabaptin-5, a multivalent adaptor protein that also binds to Rab4, Rab5 and to domains present in gamma- adaptins and the Golgi-localized, gamma-ear-containing, ARF-binding proteins (GGAs). Rabaptin-5 augments the Rabex-5 exchange activity, thus generating GTP-bound, membrane-associated Rab5 that, in turn, binds Rabaptin-5 and stabilizes the Rabex-5/Rabaptin-5 complex on endosomes. While the Rabex-5-Rabaptin-5 complex is critical to the regulation of endosomal fusion, the structural determinants of this interaction are unknown. Likewise, the possible binding and covalent attachment of ubiquitin to Rabex-5, two modifications that are critical to the function of yeast Vps9p in endosomal transport, have not been studied. In this study, we identify the 401-462 and 551-661 coiled coils as the regions in Rabex-5 and Rabaptin-5, respectively, that interact with one another. We also demonstrate that Rabex-5 undergoes ubiquitination and binds ubiquitin, though not via its proposed CUE-like domain. Instead, an alpha-helix immediately adjacent to the Rabex-5 Cys2/Cys2 zinc finger binds ubiquitin, and residues in both the zinc finger and the adjacent ubiquitin binding site are important for the in vivo ubiquitination of Rabex-5. Importantly, we demonstrate that the Rabex-5 zinc finger displays ubiquitin ligase (E3) activity. These observations extend our understanding of the regulation of Rabex-5 by Rabaptin-5. Moreover, the demonstration that Rabex-5 is an ubiquitin ligase that binds ubiquitin and undergoes ubiquitination indicates that its role in endosome fusion may be subject to additional regulation by ubiquitin-dependent modifications.

Reference Type
Journal Article
Authors
Mattera R, Tsai YC, Weissman AM, Bonifacino JS
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