Structural and biochemical studies on ataxin-1 complexes and their implication on spinocerebellar ataxia type 1 (SCA1)

Abstract

Spinocerebellar ataxia type 1 (SCA1) is one of nine poly Q diseases. Poly Q expansion of ataxin-1 (ATXN1) protein causes SCA1 disease. In the nucleus, ATXN1 has two binding partners, CIC and RBM17, which compete for ATXN1 binding. CIC binds to the AXH domain of ATXN1, and RBM17 interacts with ATXN1 by recognizing phosphorylated C-terminal 776 serine. Poly Q expansion promotes the formation of ATXN1 and RBM17 complex. The imbalance of the formation of these two complexes might contribute to pathogenesis. Here, I report about the biological implication of the crystal structure of ATXN1 and CIC complex solved by x-ray crystallography, and validation of the structure model of ATXN1 and RBM17 by biochemical studies. Firstly, I determined the crystal structure of ATXN1 AXH domain and CIC complex. In the structure, CIC peptide binds to the hydrophobic pocket of the AXH domain, disrupting the homo-dimerization of the AXH domain. Furthermore, the crystal structure revealed that the AXH domain and CIC complex was reconfigured to make a new form of dimer via CIC peptides. Next, the structure of ATXN1 and RBM17 complex was modeled using RBM17 and ULM5 crystal structure as a template. The modeled structure of ATXN1 and RBM17 complex was validated by mutagenesis studies. Furthermore, I showed that phosphorylation of C-terminal 776 serine, and 773 arginine of ATXN1 are important for RBM17 binding by biochemical approaches. These results might provide a molecular mechanism of SCA1 pathogenesis by identifying the reconfiguration of ATXN1 dimerization via CIC, and present molecular interactions between ATXN1 and RBM17.