Mov34/MPN/PAD-1 family: proteasomal regulatory protein Rpn11 and signalosome complex subunit CSN5
This family contains proteasomal regulatory protein Rpn11 (26S proteasome regulatory subunit rpn11; PAD1; POH1; RPN11; PSMD14; Rpn11 subunit of the 19S-proteasome; regulatory particle number 11) and signalosomal CSN5 (COP9 signalosome complex subunit 5; COP9 complex homolog subunit 5; c-Jun activation domain-binding protein-1; CSN5/JAB1; JAB1). COP9 signalosome (CSN) and the proteasome lid are paralogous complexes and their respective subunits CSN5 and Rpn11 are most closely related between the two complexes, both containing the conserved JAMM (JAB1/MPN/Mov34 metalloenzyme) motif involved in zinc ion coordination and providing the active site for isopeptidase activity. Rpn11 is responsible for substrate deubiquitination during proteasomal degradation. It is essential for maintaining a correct cell cycle and normal mitochondrial morphology and physiology; mutations in Rpn11 cause cell cycle and mitochondrial defects, temperature sensitivity and sensitivity to DNA damaging reagents such as UV. It has been shown that the C-terminal region of Rpn11 is involved in the regulation of the mitochondrial fission and tubulation processes. CSN5, one of the eight subunits of CSN, is critical for nuclear export and the degradation of several tumor suppressor proteins, including p53, p27, and Smad4. Its MPN+ domain is critical for the physical interaction of RUNX3 and Jab1. It has been suggested that the direct interaction of CSN5/JAB1 with p27 provides p27 with a leucine-rich nuclear export signal (NES), which is required for binding to chromosomal region maintenance 1 (CRM1), and facilitates nuclear export. The over-expression of CSN5/JAB1 also has been implicated in cancer initiation and progression, including cancer of the lung, pancreas, mouth, thyroid, and breast, suggesting that the oncogenic activity of CSN5 is associated with the down-regulation of RUNX3.