Direct Inhibition of Gcn5 Protein Catalytic Activity by Polyglutamine-expanded Ataxin-7

Background: Polyglutamine expansion of the ataxin-7 protein, a subunit of the SAGA (Spt-Ada-Gcn5-acetyltransferase) complex, causes spinocerebellar ataxia type 7. Results: Polyglutamine-expanded ataxin-7 aberrantly binds to SAGA, affecting Gcn5 acetyltransferase activity at SAGA-regulated genes. Conclusion: Ataxin-7 exhibits a dominant-negative effect on Gcn5 catalytic activity in vivo. Significance: We show a direct dominant effect of polyglutamine-expanded ataxin-7 on SAGA acetyltransferase function. Spinocerebellar ataxia type 7 (SCA7) is a neurodegenerative disease caused by polyglutamine (polyQ) expansion within the N-terminal region of the ataxin-7 protein, a known subunit of the SAGA complex. Although the mechanisms of SCA7 pathogenesis remain poorly understood, previous studies have shown perturbations in SAGA histone acetyltransferase function and transcriptional alterations. We sought to determine whether and how polyQ-expanded ataxin-7 affects SAGA catalytic activity. Here, we determined that polyQ-expanded ataxin-7 directly bound the Gcn5 catalytic core of SAGA while in association with its regulatory proteins, Ada2 and Ada3. This caused a significant decrease in Gcn5 histone acetyltransferase activity in vitro and in vivo at two SAGA-regulated galactose genes, GAL1 and GAL7. However, Gcn5 occupancy at the GAL1 and GAL7 promoters was increased in these cells, revealing a dominant-negative phenotype of the polyQ-expanded ataxin-7-incorporated, catalytically inactive SAGA. These findings suggest a dominant mechanism of polyQ-mediated SAGA inhibition that potentially contributes to SCA7 disease pathogenesis.