title: Nuclear import/export of hRPF1/Nedd4 regulates the ubiquitin-dependent degradation of its nuclear substrates.
abstract: The ubiquitin-protein ligase (E3), hRPF1/Nedd4, is a component of the ubiquitin-proteasome pathway responsible for substrate recognition and specificity. Although previously characterized as a regulator of the stability of cytoplasmic proteins, hRPF1/Nedd4 has also been suggested to have a role in the nucleus. However, in light of the cytoplasmic localization of hRPF1/Nedd4, it is unclear whether bona fide nuclear substrates of hRPF1/Nedd4 exist, and if so, what mechanism may allow a cytoplasmic ubiquitin ligase to manifest nuclear activity. Our search for nuclear substrates led to the identification of the human proline-rich transcript, brain-expressed (hPRTB) protein, the ubiquitination and degradation of which is regulated by hRPF1/Nedd4. Interestingly, hPRTB colocalizes with the splicing factor SC35 in nuclear speckles. Finally, we demonstrate that hRPF1/Nedd4 is indeed capable of entering the nucleus; however, the presence of a functional Rev-like nuclear export sequence in hRPF1/Nedd4 ensures a predominant cytoplasmic localization. Cumulatively, these findings highlight a nuclear role for the ubiquitin ligase hRPF1/Nedd4 and underscore cytoplasmic/nuclear localization as an important regulatory component of hRPF1/Nedd4-substrate recognition.
authors.affiliation: Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
authors.forename: M H
authors.initials: MH
authors.lastname: Hamilton
authors.forename: I
authors.initials: I
authors.lastname: Tcherepanova
authors.forename: J M
authors.initials: JM
authors.lastname: Huibregtse
authors.forename: D P
authors.initials: DP
authors.lastname: McDonnell
mesh_terms: D000595:Amino Acid Sequence; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism*; D056827:Endosomal Sorting Complexes Required for Transport; D006367:HeLa Cells; D006801:Humans; D008025:Ligases / Q000378:metabolism*; D008969:Molecular Sequence Data; D000075702:Nedd4 Ubiquitin Protein Ligases; D009687:Nuclear Proteins / Q000378:metabolism; D021381:Protein Transport; D013379:Substrate Specificity; D044767:Ubiquitin-Protein Ligases; D014452:Ubiquitins / Q000378:metabolism
chemical_list: D002135:Calcium-Binding Proteins; D056827:Endosomal Sorting Complexes Required for Transport; D009687:Nuclear Proteins; D014452:Ubiquitins; D000075702:Nedd4 Ubiquitin Protein Ligases; C000618483:Nedd4 protein, human; D044767:Ubiquitin-Protein Ligases; D008025:Ligases