The ataxia protein sacsin is a functional cochaperone that protects against polyglutamine expanded ataxin-1.
Hum Mol Genet. 2009 Feb 10;
Authors: Parfitt DA, Michael GJ, Vermeulen EG, Prodromou NV, Webb TR, Gallo JM, Cheetham ME, Nicoll WS, Blatch GL, Chapple JP
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 characterised 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 localisation 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 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.
PMID: 19208651 [PubMed - as supplied by publisher]