Structural basis for recognition of the third SH3 domain of full-length R85 (R85FL)/ponsin by ataxin-7

•PolyQ-expanded ataxin-7 may recruit numerous proteins to inclusion bodies.•Ataxin-7 interacts with SH3C of R85FL through its second PRR.•SH3C contains a large negatively charged surface for ataxin-7 binding.•Sequestration of R85FL by ataxin-7 is mediated by specific SH3C–PRR interaction.•Recognition of R85FL by ataxin-7 recapitulates the pathology of SCA7 disease. Ataxin-7 (Atx7) is a component of the nuclear transcription co-activator complex; its polyglutamine (polyQ) expansion may cause nuclear accumulation and recruit numerous proteins to the intranuclear inclusion bodies. Full-length R85 (R85FL) is such a protein sequestered by polyQ-expanded Atx7. Here, we report that Atx7 specifically interacts with the third SH3 domain (SH3C) of R85FL through its second portion of proline-rich region (PRR). NMR structural analysis of the SH3C domain and its complex with PRR revealed that SH3C contains a large negatively charged surface for binding with the RRTR motif of Atx7. Microscopy imaging demonstrated that sequestration of R85FL by the polyQ-expanded Atx7 in cell is mediated by this specific SH3C–PRR interaction, which is implicated in the pathogenesis of spinocerebellar ataxia 7. Atx7 PP2 and SH3Cbind by isothermal titration calorimetry (View Interaction: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12) Atx7binds to SH3C by pull down (View interaction) Atx7100Q and SH3Ccolocalize by fluorescence microscopy (View interaction) SH3C and Atx7bind by nuclear magnetic resonance (View interaction)