Metalloprotease subunit of the 19S regulatory particle; part of 26S proteasome lid; couples deubiquitination and degradation of proteasome substrates; allosteric ubiquitin sensor that slows spontaneous conformational transitions of the proteasome and aids substrate insertion into the ATPase motor to accelerate degradation up to 4-fold; involved in the fission of mitochondria and peroxisomes independent of catalytic activity; protein abundance increases in response to DNA replication stress