Quantitative Proteomics Reveals Extensive Changes in the Ubiquitinome after Perturbation of the Proteasome by Targeted dsRNA-Mediated Subunit Knockdown in Drosophila.

The ubiquitin-proteasome system (UPS), a highly regulated mechanism including the active marking of proteins by ubiquitin to be degraded, is critical in regulating proteostasis. Dysfunctioning of the UPS has been implicated in diseases such as cancer and neurodegenerative disorders. Here we investigate the effects of proteasome malfunctioning on global proteome and ubiquitinome dynamics using SILAC proteomics in Drosophila S2 cells. dsRNA-mediated knockdown of specific proteasome target subunits is used to inactivate the proteasome. Upon this perturbation, both the global proteome and the ubiquitinome become modified to a great extent, with the overall impact on the ubiquitinome being the most dramatic. The abundances of ∼10% of all proteins are increased, while the abundances of the far majority of over 14 000 detected diGly peptides are increased, suggesting that the pool of ubiquitinated proteins is highly dynamic. Remarkably, several proteins show heterogeneous ubiquitination dynamics, with different lysine residues on the same protein showing either increased or decreased ubiquitination. This suggests the occurrence of simultaneous and functionally different ubiquitination events. This strategy offers a powerful tool to study the response of the ubiquitinome upon interruption of normal UPS activity by targeted interference and opens up new avenues for the dissection of the mode of action of individual components of the proteasome. Because this is to our knowledge the first comprehensive ubiquitinome screen upon proteasome malfunctioning in a fruit fly cell system, this data set will serve as a valuable repository for the Drosophila community.