To degrade or release: ubiquitin-chain remodeling

The proteasome controls many cellular processes by degrading a large number of regulatory proteins. Most proteins are targeted to the proteasome through covalent tagging by a chain consisting of several copies of the small protein ubiquitin. Finley and coworkers have now discovered two proteins, Hul5 and Ubp6, which regulate degradation further, when bound to the proteasome. Hul5 promotes degradation by extending the number of ubiquitin moieties in the tag on substrates, whereas Ubp6 antagonizes degradation by trimming ubiquitin from the tag. The balance between these two opposing activities might control the substrate specificity of the proteasome and adjusting the balance would provide a new level of degradation control.