title: Defective Proteasome Delivery of Polyubiquitinated Proteins by Ubiquilin-2 Proteins Containing ALS Mutations.
abstract: Ubiquilin proteins facilitate delivery of ubiquitinated proteins to the proteasome for degradation. Interest in the proteins has been heightened by the discovery that gene mutations in UBQLN2 cause dominant inheritance of amyotrophic lateral sclerosis (ALS). However, the mechanisms by which the mutations cause ALS are not known. Here we report on the underlying defect of ubiquilin-2 proteins containing ALS-linked mutations in affecting proteasome-mediated degradation. We found that overexpression of ubiquilin-2 proteins containing any one of five different ALS mutations slow degradation of Myc, a prototypic proteasome substrate. Examination of coprecipitating proteins indicated that the mutant proteins are generally capable of binding polyubiquitinated proteins, but defective in binding the proteasome. GST-pulldown studies revealed that many of the mutants bind weaker to the S5a subunit of the proteasome, compared with wild type (WT) ubiquilin-2 protein. The results suggest the mutant proteins are unable to deliver their captured cargo to the proteasome for degradation, which presumably leads to toxicity. Quantification of cell death is consistent with this idea. Measurement of protein turnover further indicated the mutant proteins have longer half-lives than WT ubiquilin-2. Our studies provide novel insight into the mechanism by which ALS-linked mutations in UBQLN2 interfere with protein degradation.
authors.affiliation: Center for Biomedical Engineering and Technology, University of Maryland School of Medicine, Baltimore, Maryland, United States of America.
authors.forename: Lydia
authors.initials: L
authors.lastname: Chang
authors.affiliation: Center for Biomedical Engineering and Technology, University of Maryland School of Medicine, Baltimore, Maryland, United States of America; Department of Anatomy and Neurobiology, University of Maryland School of Medicine, Baltimore, Maryland, United States of America.
authors.forename: Mervyn J
authors.initials: MJ
authors.lastname: Monteiro
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000690:Amyotrophic Lateral Sclerosis / Q000235:genetics*; D000071183:Autophagy-Related Proteins; D015153:Blotting, Western; D018797:Cell Cycle Proteins / Q000235:genetics* / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D006801:Humans; D047468:Immunoprecipitation; D002479:Inclusion Bodies; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation / Q000235:genetics*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D059748:Proteolysis; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism; D014407:Tumor Cells, Cultured; D054875:Ubiquitination; D014452:Ubiquitins / Q000235:genetics* / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D000071183:Autophagy-Related Proteins; D018797:Cell Cycle Proteins; C489427:MYC protein, human; D050505:Mutant Proteins; D016271:Proto-Oncogene Proteins c-myc; C405662:UBQLN2 protein, human; D014452:Ubiquitins; D046988:Proteasome Endopeptidase Complex