Abstract
Ubiquitination is a post-translational modification that regulates cellular processes by altering the interactions of proteins to which ubiquitin, a small protein adduct, is conjugated. Ubiquitination yields various products, including mono- and poly-ubiquitinated substrates, as well as unanchored poly-ubiquitin chains whose accumulation is considered toxic. We previously showed that transgenic, unanchored poly-ubiquitin is not problematic in Drosophila melanogaster. In the fruit fly, free chains exist in various lengths and topologies and are degraded by the proteasome; they are also conjugated onto other proteins as one unit, eliminating them from the free ubiquitin chain pool. Here, to further explore the notion of unanchored chain toxicity, we examined when free poly-ubiquitin might become problematic. We found that unanchored chains can be highly toxic if they resemble linear poly-ubiquitin that cannot be modified into other topologies. These species upregulate NF-κB signaling, and modulation of the levels of NF-κB components reduces toxicity. In additional studies, we show that toxicity from untethered, linear chains is regulated by isoleucine 44, which anchors a key interaction site for ubiquitin. We conclude that free ubiquitin chains can be toxic, but only in uncommon circumstances, such as when the ability of cells to modify and regulate them is markedly restricted.
Highlights
Post-translational modification adds a critical layer of complexity to the genome, allowing for regulation of many cellular functions
We found stark differences in the toxicity of and response to ubiquitination-resistant Ub6 chains in vivo
We previously examined whether expression of unanchored poly-Ub is toxic in Drosophila
Summary
Post-translational modification adds a critical layer of complexity to the genome, allowing for regulation of many cellular functions. Methylation of histones in nucleosomes controls the expression of specific genes [1], phosphorylation of enzymes determines their activity [2], and glycosylation affects a protein’s conformation and ability to form complexes [3]. A protein’s ubiquitination status can lead to various outcomes, from alteration of its protein–protein interactions to changes in its localization, function, enzymatic activity and half-life [5,6,7,8,9]. Ubiquitination is a sequential, ATP-dependent process that requires a Ub-activating enzyme (E1), a Ub-conjugating enzyme (E2), and a Ub ligase (E3) to covalently attach a Ub molecule to a substrate protein, typically at one of the substrate’s lysine residues [5]. With seven lysine residues of its own, Cells 2020, 9, 1519; doi:10.3390/cells9061519 www.mdpi.com/journal/cells
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have