Proteomic Analysis of Wild-type and Mutant Huntingtin-associated Proteins in Mouse Brains Identifies Unique Interactions and Involvement in Protein Synthesis

Huntington disease is a neurodegenerative disorder caused by a CAG repeat amplification in the gene huntingtin (HTT) that is reflected by a polyglutamine expansion in the Htt protein. Nearly 20 years of research have uncovered roles for Htt in a wide range of cellular processes, and many of these discoveries stemmed from the identification of Htt-interacting proteins. However, no study has employed an impartial and comprehensive strategy to identify proteins that differentially associate with full-length wild-type and mutant Htt in brain tissue, the most relevant sample source to the disease condition. We analyzed Htt affinity-purified complexes from wild-type and HTT mutant juvenile mouse brain from two different biochemical fractions by tandem mass spectrometry. We compared variations in protein spectral counts relative to Htt to identify those proteins that are the most significantly contrasted between wild-type and mutant Htt purifications. Previously unreported Htt interactions with Myo5a, Prkra (PACT), Gnb2l1 (RACK1), Rps6, and Syt2 were confirmed by Western blot analysis. Gene Ontology analysis of these and other Htt-associated proteins revealed a statistically significant enrichment for proteins involved in translation among other categories. Furthermore, Htt co-sedimentation with polysomes in cytoplasmic mouse brain extracts is dependent upon the presence of intact ribosomes. Finally, wild-type or mutant Htt overexpression inhibits cap-dependent translation of a reporter mRNA in an in vitro system. Cumulatively, these data support a new role for Htt in translation and provide impetus for further study into the link between protein synthesis and Huntington disease pathogenesis.