title: The ataxia protein sacsin is a functional co-chaperone that protects against polyglutamine-expanded ataxin-1.
abstract: An extensive protein-protein interaction network has been identified between proteins implicated in inherited ataxias. The protein sacsin, which is mutated in the early-onset neurodegenerative disease autosomal recessive spastic ataxia of Charlevoix-Saguenay, is a node in this interactome. Here, we have established the neuronal expression of sacsin and functionally characterized domains of the 4579 amino acid protein. Sacsin is most highly expressed in large neurons, particularly within brain motor systems, including cerebellar Purkinje cells. Its subcellular localization in SH-SY5Y neuroblastoma cells was predominantly cytoplasmic with a mitochondrial component. We identified a putative ubiquitin-like (UbL) domain at the N-terminus of sacsin and demonstrated an interaction with the proteasome. Furthermore, sacsin contains a predicted J-domain, the defining feature of DnaJ/Hsp40 proteins. Using a bacterial complementation assay, the sacsin J-domain was demonstrated to be functional. The presence of both UbL and J-domains in sacsin suggests that it may integrate the ubiquitin-proteasome system and Hsp70 function to a specific cellular role. The Hsp70 chaperone machinery is an important component of the cellular response towards aggregation prone mutant proteins that are associated with neurodegenerative diseases. We therefore investigated the effects of siRNA-mediated sacsin knockdown on polyglutamine-expanded ataxin-1. Importantly, SACS siRNA did not affect cell viability with GFP-ataxin-1[30Q], but enhanced the toxicity of GFP-ataxin-1[82Q], suggesting that sacsin is protective against mutant ataxin-1. Thus, sacsin is an ataxia protein and a regulator of the Hsp70 chaperone machinery that is implicated in the processing of other ataxia-linked proteins.
authors.affiliation: William Harvey Research Institute, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, UK.
authors.forename: David A
authors.initials: DA
authors.lastname: Parfitt
authors.forename: Gregory J
authors.initials: GJ
authors.lastname: Michael
authors.forename: Esmeralda G M
authors.initials: EG
authors.lastname: Vermeulen
authors.forename: Natalia V
authors.initials: NV
authors.lastname: Prodromou
authors.forename: Tom R
authors.initials: TR
authors.lastname: Webb
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Gallo
authors.forename: Michael E
authors.initials: ME
authors.lastname: Cheetham
authors.forename: William S
authors.initials: WS
authors.lastname: Nicoll
authors.forename: Gregory L
authors.initials: GL
authors.lastname: Blatch
authors.forename: J Paul
authors.initials: JP
authors.lastname: Chapple
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001259:Ataxia / Q000235:genetics / Q000378:metabolism*; D000067696:Ataxin-1; D000067528:Ataxins; D045744:Cell Line, Tumor; D018840:HSP70 Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D006360:Heat-Shock Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008297:Male; D018832:Molecular Chaperones / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D009474:Neurons / Q000378:metabolism; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D010455:Peptides / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D051381:Rats; D017208:Rats, Wistar; D016415:Sequence Alignment
chemical_list: C000593749:ATXN1 protein, human; D000067696:Ataxin-1; D000067528:Ataxins; C000593751:Atxn1 protein, rat; D018840:HSP70 Heat-Shock Proteins; D006360:Heat-Shock Proteins; D018832:Molecular Chaperones; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins; D010455:Peptides; C404403:SACS protein, human; C541920:SACS protein, rat; C097188:polyglutamine