Date Published: December 23, 2009
Publisher: Public Library of Science
Author(s): Cesira de Chiara, Rajesh P. Menon, Molly Strom, Toby J. Gibson, Annalisa Pastore, Yue Feng. http://doi.org/10.1371/journal.pone.0008372
Abstract: Ataxin-1 (Atx1), a member of the polyglutamine (polyQ) expanded protein family, is responsible for spinocerebellar ataxia type 1. Requirements for developing the disease are polyQ expansion, nuclear localization and phosphorylation of S776. Using a combination of bioinformatics, cell and structural biology approaches, we have identified a UHM ligand motif (ULM), present in proteins associated with splicing, in the C-terminus of Atx1 and shown that Atx1 interacts with and influences the function of the splicing factor U2AF65 via this motif. ULM comprises S776 of Atx1 and overlaps with a nuclear localization signal and a 14-3-3 binding motif. We demonstrate that phosphorylation of S776 provides the molecular switch which discriminates between 14-3-3 and components of the spliceosome. We also show that an S776D Atx1 mutant previously designed to mimic phosphorylation is unsuitable for this aim because of the different chemical properties of the two groups. Our results indicate that Atx1 is part of a complex network of interactions with splicing factors and suggest that development of the pathology is the consequence of a competition of aggregation with native interactions. Studies of the interactions formed by non-expanded Atx1 thus provide valuable hints for understanding both the function of the non-pathologic protein and the causes of the disease.
Partial Text: Ataxin-1 (Atx1) is a 98 kDa protein and a member of the protein family containing polymorphic polyglutamine (polyQ) tracts related to neurodegenerative diseases –. Although clinically distinct, these pathologies are all caused by a common mechanism: when the polyQ tract is anomalously expanded above a threshold which varies for each disease the polyQ carrier protein misfolds and aggregates leading to cellular death. Expansion in Atx1 above 35–42 glutamines is associated with spinocerebellar ataxia type 1 (SCA1), an autosomal dominant neurodegenerative disorder characterized by motor coordination deficits caused by progressive loss of Purkinje cells in the cerebellar cortex and neurons in the brain stem and spinocerebellar tracts.
It is common when studying proteins associated with diseases to consider them mainly under the aspect of how they are linked to pathology. This is particularly the case for polyQ proteins: much has been said about their relationship to aggregation and cellular toxicity but still little is known about their cellular functions when they are in their non-pathologic state. In this study, we have followed a different approach. We focused on the functional role of non-pathologic Atx1 and set out to identify sequence motifs which could provide direct information about the mechanisms involved in recognition.