Ubiquitin-mediated sequestration of normal cellular proteins into polyglutamine aggregates

Departments of ^*Cancer and Cell Biology and ^‡Bioinformatics, Genomics Institute of the Novartis Research Foundation (GNF), 10675 John J. Hopkins Drive, San Diego, CA 92121; and ^§Gene Expression Laboratory, The Salk Institute, La Jolla, CA 92037

Edited by Peter M. Howley, Harvard Medical School, Boston, MA, and approved May 23, 2003 (received for review January 13, 2003)

Abstract

A hallmark of most neurodegenerative diseases, including those caused by polyglutamine expansion, is the formation of ubiquitin (Ub)-positive protein aggregates in affected neurons. This finding suggests that the Ub system may be involved in common mechanisms underlying these otherwise unrelated diseases. Here we report the finding of ataxin-3 (Atx-3), whose mutation is implicated in the neurodegenerative disease spinocerebellar ataxia type 3, in a bioinformatics search of the human genome for components of the Ub system. We show that wild-type Atx-3 is a Ub-binding protein and that the interaction of Atx-3 with Ub is mediated by motifs homologous to those found in a proteasome subunit. Both wild-type Atx-3 and the otherwise unrelated Ub-binding protein p62/Sequestosome-1 have been shown to be sequestered into aggregates in affected neurons in several neurodegenerative diseases, but the mechanism for this recruitment has remained unclear. In this article, we show that functional Ub-binding motifs in Atx-3 and p62 proteins are required for the localization of both proteins into aggregates in a cell-based assay that recapitulates several features of polyglutamine disease. We propose that the Ub-mediated sequestration of essential Ub-binding protein(s) into aggregates may be a common mechanism contributing to the pathogenesis of neurodegenerative diseases.

Footnotes

↵ ∥ To whom correspondence should be addressed. E-mail: joazeiro{at}gnf.org.
↵ † K.M.D. and W.L. contributed equally to this work.
↵ ¶ Present address: Department of Physiology and Biophysics, University of Medicine and Dentistry of New Jersey–Robert Wood Johnson Medical School, 675 Hoes Lane, Piscataway, NJ 08854.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: SCA, spinocerebellar ataxia; Atx-3, ataxin-3; Ub, ubiquitin; PUB, polyUb-binding; PUBH, PUB-homologous; polyQ, polyglutamine; UIM, Ub-interaction motif; UBA, Ub-associated; HMM, hidden Markov model; MJD, Machado–Joseph's disease; APC, anaphase-promoting complex; CFP, cyan fluorescent protein.