title: Monoubiquitination of survival motor neuron regulates its cellular localization and Cajal body integrity.
abstract: Low levels of the survival motor neuron (SMN) protein cause spinal muscular atrophy, the leading genetic disorder for infant mortality. SMN is ubiquitously expressed in various cell types and localizes in both the cytoplasm and the nucleus, where it concentrates in two subnuclear structures termed Cajal body (CB) and gems. In addition, SMN can also be detected in the nucleolus of neurons. Mechanisms that control SMN sorting in the cell remain largely unknown. Here, we report that the ubiquitin (Ub) ligase Itch directly interacts with and monoubiquitinates SMN. Monoubiquitination of SMN has a mild effect on promoting proteasomal degradation of SMN. We generated two SMN mutants, SMN(K0), in which all lysines are mutated to arginines and thereby abolishing SMN ubiquitination, and Ub-SMN(K0), in which a single Ub moiety is fused at the N-terminus of SMN(K0) and thereby mimicking SMN monoubiquitination. Immunostaining assays showed that SMN(K0) mainly localizes in the nucleus, whereas Ub-SMN(K0) localizes in both the cytoplasm and the nucleolus in neuronal SH-SY5Y cells. Interestingly, canonical CB foci and coilin/small nuclear ribonucleoprotein (snRNP) co-localization are significantly impaired in SH-SY5Y cells stably expressing SMN(K0) or Ub-SMN(K0). Thus, our studies discover that Itch monoubiquitinates SMN and monoubiquitination of SMN plays an important role in regulating its cellular localization. Moreover, mislocalization of SMN disrupts CB integrity and likely impairs snRNP maturation.
authors.affiliation: Department of Biochemistry and Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80015, USA and.
authors.forename: Ke-Jun
authors.initials: KJ
authors.lastname: Han
authors.affiliation: Department of Biochemistry and Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80015, USA and.
authors.forename: Daniel
authors.initials: D
authors.lastname: Foster
authors.affiliation: Department of Oncology, Johns Hopkins School of Medicine, Baltimore, MD 21287, USA.
authors.forename: Edward W
authors.initials: EW
authors.lastname: Harhaj
authors.affiliation: Department of Biochemistry and Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80015, USA and.
authors.forename: Monika
authors.initials: M
authors.lastname: Dzieciatkowska
authors.affiliation: Department of Biochemistry and Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80015, USA and.
authors.forename: Kirk
authors.initials: K
authors.lastname: Hansen
authors.affiliation: Department of Biochemistry and Molecular Genetics, University of Colorado School of Medicine, Aurora, CO 80015, USA and changwei.liu@ucdenver.edu.
authors.forename: Chang-Wei
authors.initials: CW
authors.lastname: Liu
mesh_terms: D000818:Animals; D020541:Coiled Bodies / Q000378:metabolism*; D006801:Humans; D051379:Mice; D009134:Muscular Atrophy, Spinal / Q000378:metabolism; D021381:Protein Transport; D012097:Repressor Proteins / Q000378:metabolism*; D055533:Survival of Motor Neuron 1 Protein / Q000737:chemistry / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism*; D054875:Ubiquitination
chemical_list: D012097:Repressor Proteins; C526471:SMN1 protein, human; C526472:Smn1 protein, mouse; D055533:Survival of Motor Neuron 1 Protein; C432966:ITCH protein, human; C471637:Itch protein, mouse; D044767:Ubiquitin-Protein Ligases