USP14 deubiquitinates proteasome-bound substrates that are ubiquitinated at multiple sites.

The proteasome-associated enzyme USP14 regulates protein degradation by removing ubiquitin from proteins; here it is shown that USP14 removes ubiquitin chains from in vitro generated cyclin B conjugates en bloc and within milliseconds, before the proteasome has a chance to initiate degradation, and proceeds until a single chain remains.