Functional division of substrate processing cofactors of the ubiquitin-selective Cdc48 chaperone.

Ubiquitin-dependent protein degradation usually involves escort factors that target ubiquitylated substrates to the proteasome. A central element in a major escort pathway is Cdc48, a chaperone-like AAA ATPase that collects ubiquitylated substrates via alternative substrate-recruiting cofactors. Cdc48 also associates with Ufd2, an E4 multiubiquitylation enzyme that adds further ubiquitin moieties to preformed ubiquitin conjugates to promote degradation. Here, we show that E4 can be counteracted in vivo by two distinct mechanisms. First, Ufd3, a WD40 repeat protein, directly competes with Ufd2, because both factors utilize the same docking site on Cdc48. Second, Cdc48 also binds Otu1, a deubiquitylation enzyme, which disassembles multiubiquitin chains. Notably, Cdc48 can bind Otu1 and Ufd3 simultaneously, making a cooperation of both inhibitory mechanisms possible. We propose that the balance between the distinct substrate-processing cofactors may determine whether a substrate is multiubiquitylated and routed to the proteasome for degradation or deubiquitylated and/or released for other purposes.