Protein ubiquitylation is a dynamic process that affects the function and stability of
proteins and controls essential cellular processes ranging from cell proliferation to cell death. This process is regulated through the balanced action of E3
ubiquitin ligases and deubiquitylating
enzymes (DUB) which conjugate
ubiquitins to, and remove them from target
proteins, respectively. Our genetic analysis has revealed that the deubiquitylating
enzyme DmUsp5 is required for maintenance of the
ubiquitin equilibrium, cell survival and normal development in Drosophila. Loss of the DmUsp5 function leads to late larval lethality accompanied by the induction of apoptosis. Detailed analyses at a cellular level demonstrated that DmUsp5 mutants carry
multiple abnormalities, including a drop in the free monoubiquitin level, the excessive accumulation of free polyubiquitins, polyubiquitylated
proteins and subunits of the
26S proteasome. A shortage in free
ubiquitins results in the induction of a
ubiquitin stress response previously described only in the unicellular budding yeast. It is characterized by the induction of the
proteasome-associated deubiquitylase DmUsp14 and sensitivity to
cycloheximide. Removal of DmUsp5 also activates the pro-apoptotic machinery thereby resulting in widespread apoptosis, indicative of an anti-apoptotic role of DmUsp5. Collectively, the pleiotropic effects of a loss of DmUsp5 function can be explained in terms of the existence of a limited pool of free monoubiquitins which makes the
ubiquitin-dependent processes mutually interdependent.