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Human Molecular Genetics Advance Access published online on July 5, 2006
Human Molecular Genetics, doi:10.1093/hmg/ddl164
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© The Author 2006. Published by Oxford University Press. All rights reserved
Received May 3, 2006
Revised June 28, 2006
Accepted June 28, 2006

Article

Ataxin-3 binds VCP/p97 and regulates retrotranslocation of ERAD substrates

Xiaoyan Zhong 1 and Randall N. Pittman 1 *

1 Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, PA19104-6084

* To whom correspondence should be addressed.
Randall N. Pittman, E-mail: pittman{at}pharm.med.upenn.edu


  Abstract

Expansion of a polyglutamine tract in ataxin-3 (AT3) results in spinocerebellar ataxia type 3/Machado-Joseph disease, one of nine polyglutamine neurodegenerative diseases. Understanding the normal functions of AT3 as well as its function in the context of expansion of the polyglutamine tract is critical for understanding the disease process. AT3 is a deubiquitylating enzyme with limited information on its cellular functions. We find that transfecting cells with AT3 increases cellular levels of ER-associated degradation (ERAD) substrates, CD3{delta} and TCR{alpha}, but does not alter levels of several non-ERAD substrates. AT3 increases the level of CD3{delta} by decreasing its degradation; pathogenic AT3 decreases degradation to a greater extent than wild-type AT3. Knock-down of endogenous AT3 decreases levels of CD3{delta}?suggesting that a normal function of AT3 is to regulate levels of ERAD substrates. AT3 binds VCP/p97, a key protein responsible for extracting ERAD substrates from the ER; binding is modulated by the size of the polyglutamine tract and mutating a sequence adjacent to the polyglutamine tract inhibits the AT3-VCP interaction and AT3-dependent accumulation of CD3{delta}. AT3 and Ufd1 bind VCP in a mutually exclusive manner; AT3 decreases the interaction of VCP with Ufd1 as well as with ubiquitylated proteins. Using a reconstituted system, AT3 inhibits retrotranslocation of an ERAD substrate from the ER. These data suggest that a normal function of AT3 is to regulate flow through the ERAD pathway by modulating VCP-dependent extraction of proteins from the ER.


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