The polyglutamine neurodegenerative protein ataxin-3 binds polyubiquitylated proteins and has ubiquitin protease activity.
The ubiquitin-proteasome pathway is critically involved in the pathology of neurodegenerative diseases characterized by protein misfolding and aggregation. Data in the present study suggest that the polyglutamine neurodegenerative disease protein, ataxin-3 (AT3), functions in the ubiquitin-proteasome pathway. AT3 contains an ubiquitin interaction motif (UIM) domain that binds polyubiquitylated proteins with a strong preference for chains containing four or more ubiquitins. Mutating the conserved leucine in the first UIM (L229A) almost totally eliminates binding to polyubiquitin chains while a similar mutation in the second UIM (L249A) also inhibits binding to polyubiquitin chains but to a lesser extent. Both wild-type and pathological AT3 increase cellular levels of a short-lived GFP that is degraded by the ubiquitin-proteasome pathway. AT3 has several properties characteristic of ubiquitin proteases including decreasing polyubiquitylation of 125I-lysozyme by removing ubiquitin from polyubiquitin chains, cleaving a ubiquitin protease substrate, and binding the specific ubiquitin protease inhibitor, ubiquitin-aldehyde. Mutating the predicted catalytic cysteine in AT3 inhibits each of these ubiquitin protease activities. The ability to bind and cleave ubiquitylated proteins is consistent with AT3 playing a role in the ubiquitin-proteasome system. This raises the possibility that pathological AT3, which tends to misfold and aggregate, may be exposed to aggregate-prone misfolded/denatured proteins as part of its normal function.
Burnett
Barrington
B
Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104-6084, USA.
Li
Fusheng
F
Pittman
Randall N
RN
D000818:Animals; D000067699:Ataxin-3; D001665:Binding Sites; D016466:CHO Cells; D002478:Cells, Cultured; D006224:Cricetinae; D003412:Cricetulus; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics; D009097:Multienzyme Complexes / Q000378:metabolism; D009113:Muramidase / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D009687:Nuclear Proteins; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D011993:Recombinant Fusion Proteins / Q000378:metabolism*; D012097:Repressor Proteins; D025801:Ubiquitin / Q000737:chemistry / Q000378:metabolism*
D009097:Multienzyme Complexes; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; D025801:Ubiquitin; D009113:Muramidase; C092341:ATXN3 protein, human; D000067699:Ataxin-3; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex