In 2000 we discovered that the p97, and its conserved yeast homologue Cdc48, is an essential component of ERAD that takes part in the elimination of virtually all ERAD substrates (Rabinovich et al., 2002). The p97/Cdc48 is a cytosolic homo-hexameric AAA-ATPase that functions as a molecular machine.  Initially, we found p97 in association with a luminal ERAD substrate and subsequently demonstrated the role of its homologue Cdc48 in the degradation in yeast of two well-established ERAD substrates, membrane 6myc-Hmg2 and the luminal CPY* (Rabinovich et al. 2002). To date, p97/Cdc48 is a hallmark of ERAD, although it also participates in several other cellular functions (Bar-Nun, 2005). Being an ATPase that resides in the cytosol, p97/Cdc48 provides the driving force for dislocating ERAD substrates from the ER back to the cytosol, as the stabilized luminal ERAD substrate CPY* remained trapped within the ER lumen in the temperature-sensitive cdc48-10  mutant (Elkabetz et al., 2004).

 

Interrelations between Cdc48 and proteasomal AAA-ATPases: The 26S proteasome complex is composed of a 20S proteolytic chamber and a 19S regulatory cap. Several AAA-ATPases (known in yeast as Rpt1-6) form a hetero-hexameric ring at the base of this 19S regulatory particle. We have shown that the proteolytic activity of the proteasome is dispensable for the dislocation a luminal ERAD substrate (Elkabetz et al., 2004) but two of the 19S regulatory particle subunits, Rpt2 and Rpt4, are essential for ERAD. While Rpt2 is required for degradation of every proteasomal substrate, since it gates the entry of substrates into the proteolytic chamber, Rpt4 is essential for the elimination only of ERAD substrates and is dispensable for degradation of cytosolic proteins (Lipson et al., 2008). Interestingly, Rpt4 is involved in dislocation of ERAD substrates, a role already assigned to Cdc48 (Elkabetz et al., 2004). Further experiments on the interrelations between these two AAA-ATPases suggest that Cdc48 extracts the substrate from the ER, while Rpt4 is required for transferring the substrate from Cdc48 to the proteasome (Lipson et al., 2008). We also studied the gating of the 20S catalytic particle and showed that degradation of cytosolic and ERAD substrates was similarly accelerated upon truncation of the N-termini of the 20S subunits a3 and a7 known to gate the proteolytic chamber (Rabinovich et al., 2006).