Protein degradation in eukaryotes usually requires multiubiquitylation and subsequent delivery of the tagged substrates to the proteasome. Recent studies suggest the involvement of the AAA ATPase CDC48, its cofactors, and other ubiquitin binding factors in protein degradation, but how these proteins work together is unclear. Here we show that these factors cooperate sequentially through protein-protein interactions and thereby escort ubiquitin-protein conjugates to the proteasome. Central to this pathway is the chaperone CDC48/p97, which coordinates substrate recruitment, E4-catalyzed multiubiquitin chain assembly, and proteasomal targeting. Concomitantly, CDC48 prevents the formation of excessive multiubiquitin chain sizes that are surplus to requirements for degradation. In yeast, this escort pathway guides a transcription factor from its activation in the cytosol to its final degradation and also mediates proteolysis at the endoplasmic reticulum by the ERAD pathway.

SUMMARY Mating induces changes in the receptivity and egg-laying behavior in Drosophila females, primarily due to a peptide pheromone called sex peptide which is transferred with the sperm into the female reproductive tract during copulation. Whereas sex peptide is generally believed to modulate fruitless-GAL4-expressing neurons in the central nervous system to produce behavioral changes, we found that six to eight sensory neurons on the reproductive tract labeled by both ppk-GAL4 and fruitless-GAL4 can sense sex peptide to control the induction of postmating behaviors. In these sensory neurons, sex peptide appears to act through Pertussis toxin-sensitive G proteins and suppression of protein kinase A activity to reduce synaptic output. Our results uncover a neuronal mechanism by which sex peptide exerts its control over reproductive behaviors in Drosophila females.

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.