Updated project metadata. Huntington's disease (HD) is characterized by the aggregation of polyglutamine-expanded huntingtin (HTT), proceeding from soluble oligomers to end-stage inclusions. The molecular mechanisms of how protein aggregation leads to neuronal dysfunction are not well understood. We employed mass spectrometry-based quantitative proteomics to dissect spatiotemporal mechanisms of neurodegeneration using the R6/2 mouse model of HD. We show that extensive remodeling of the soluble brain proteome correlates with changes in insoluble aggregate formation during disease progression. In-depth characterization of HTT inclusion bodies uncovered an unprecedented complexity of several hundred proteins. Sequestration to inclusions was dependent on protein expression levels and the presence of aggregation-prone amino acid sequence features, such as low-complexity regions or coiled-coil domains. Overexpression of several sequestered proteins ameliorated HTT toxicity and modified the aggregation behavior in an in vitro model of HD. Our study provides a comprehensive and spatiotemporally-resolved proteome resource of HD progression, indicating that widespread loss of protein function contributes to aggregate-mediated toxicity.