title: A family of mammalian E3 ubiquitin ligases that contain the UBR box motif and recognize N-degrons.
abstract: A subset of proteins targeted by the N-end rule pathway bear degradation signals called N-degrons, whose determinants include destabilizing N-terminal residues. Our previous work identified mouse UBR1 and UBR2 as E3 ubiquitin ligases that recognize N-degrons. Such E3s are called N-recognins. We report here that while double-mutant UBR1(-/-) UBR2(-/-) mice die as early embryos, the rescued UBR1(-/-) UBR2(-/-) fibroblasts still retain the N-end rule pathway, albeit of lower activity than that of wild-type fibroblasts. An affinity assay for proteins that bind to destabilizing N-terminal residues has identified, in addition to UBR1 and UBR2, a huge (570 kDa) mouse protein, termed UBR4, and also the 300-kDa UBR5, a previously characterized mammalian E3 known as EDD/hHYD. UBR1, UBR2, UBR4, and UBR5 shared a approximately 70-amino-acid zinc finger-like domain termed the UBR box. The mammalian genome encodes at least seven UBR box-containing proteins, which we propose to call UBR1 to UBR7. UBR1(-/-) UBR2(-/-) fibroblasts that have been made deficient in UBR4 as well (through RNA interference) were significantly impaired in the degradation of N-end rule substrates such as the Sindbis virus RNA polymerase nsP4 (bearing N-terminal Tyr) and the human immunodeficiency virus type 1 integrase (bearing N-terminal Phe). Our results establish the UBR box family as a unique class of E3 proteins that recognize N-degrons or structurally related determinants for ubiquitin-dependent proteolysis and perhaps other processes as well.
authors.affiliation: Center for Pharmacogenetics and Department of Pharmaceutical Sciences, University of Pittsburgh, PA 15261, USA.
authors.forename: Takafumi
authors.initials: T
authors.lastname: Tasaki
authors.forename: Lubbertus C F
authors.initials: LC
authors.lastname: Mulder
authors.forename: Akihiro
authors.initials: A
authors.lastname: Iwamatsu
authors.forename: Min Jae
authors.initials: MJ
authors.lastname: Lee
authors.forename: Ilia V
authors.initials: IV
authors.lastname: Davydov
authors.forename: Alexander
authors.initials: A
authors.lastname: Varshavsky
authors.forename: Mark
authors.initials: M
authors.lastname: Muesing
authors.forename: Yong Tae
authors.initials: YT
authors.lastname: Kwon
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D015152:Blotting, Northern; D002148:Calmodulin-Binding Proteins; D002478:Cells, Cultured; D004247:DNA / Q000378:metabolism; D018076:DNA, Complementary / Q000378:metabolism; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D005838:Genotype; D019427:HIV Integrase / Q000378:metabolism; D007700:Kinetics; D016086:Lentivirus / Q000201:enzymology / Q000235:genetics; D013058:Mass Spectrometry; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D008869:Microtubule-Associated Proteins / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*; D008954:Models, Biological; D008957:Models, Genetic; D008969:Molecular Sequence Data; D005810:Multigene Family; D009154:Mutation; D010455:Peptides / Q000737:chemistry; D010802:Phylogeny; D010957:Plasmids / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011485:Protein Binding; D014176:Protein Biosynthesis; D017434:Protein Structure, Tertiary; D034622:RNA Interference; D012190:Retroviridae / Q000235:genetics; D017386:Sequence Homology, Amino Acid; D012845:Sindbis Virus / Q000235:genetics; D013997:Time Factors; D014018:Tissue Distribution; D014158:Transcription, Genetic; D044767:Ubiquitin-Protein Ligases / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*; D016335:Zinc Fingers
chemical_list: D002148:Calmodulin-Binding Proteins; D018076:DNA, Complementary; D008869:Microtubule-Associated Proteins; D010455:Peptides; D004247:DNA; C113474:UBR1 protein, human; C499858:UBR2 protein, human; D044767:Ubiquitin-Protein Ligases; C528904:Ubr4 protein, mouse; D019427:HIV Integrase; D012321:DNA-Directed RNA Polymerases; D046988:Proteasome Endopeptidase Complex