Abstract
Huntington's disease is caused by a polyglutamine repeat expansion in the huntingtin protein which affects the function and folding of the protein, and results in intracellular protein aggregates. Here, we examined whether this mutation leads to altered ubiquitination of huntingtin and other proteins in both soluble and insoluble fractions of brain lysates of the Q175 knock-in Huntington's disease mouse model and the Q20 wild-type mouse model. Ubiquitination sites are detected by identification of Gly-Gly (diGly) remnant motifs that remain on modified lysine residues after digestion. We identified K6, K9, K132, K804, and K837 as endogenous ubiquitination sites of soluble huntingtin, with wild-type huntingtin being mainly ubiquitinated at K132, K804, and K837. Mutant huntingtin protein levels were strongly reduced in the soluble fraction whereas K6 and K9 were mainly ubiquitinated. In the insoluble fraction increased levels of huntingtin K6 and K9 diGly sites were observed for mutant huntingtin as compared with wild type. Besides huntingtin, proteins with various roles, including membrane organization, transport, mRNA processing, gene transcription, translation, catabolic processes and oxidative phosphorylation, were differently expressed or ubiquitinated in wild-type and mutant huntingtin brain tissues. Correlating protein and diGly site fold changes in the soluble fraction revealed that diGly site abundances of most of the proteins were not related to protein fold changes, indicating that these proteins were differentially ubiquitinated in the Q175 mice. In contrast, both the fold change of the protein level and diGly site level were increased for several proteins in the insoluble fraction, including ubiquitin, ubiquilin-2, sequestosome-1/p62 and myo5a. Our data sheds light on putative novel proteins involved in different cellular processes as well as their ubiquitination status in Huntington's disease, which forms the basis for further mechanistic studies to understand the role of differential ubiquitination of huntingtin and ubiquitin-regulated processes in Huntington's disease.