The COP9 (Constitutive photomorphogenesis 9) signalosome (CSN), a large multiprotein complex that resembles the 19S lid of the 26S proteasome, plays a central role in the regulation of the E3-cullin RING ubiquitin ligases (CRLs) [1]. The catalytic activity of the CSN complex is carried by subunit 5 (CSN5), also known as c-Jun activation domain-binding protein-1 (Jab1) [1]. This entry is the C-terminal domain found in CSN5 proteins. CSN5, whose two C-terminal helices form an antiparallel hairpin, inserts its final C-terminal helix (helix II) into the central CSN6 framework at the core of the bundle. Deletion of the C-terminal helices has a pronounced effect on CSN integrity [2].

1: Insights into the regulation of the human COP9 signalosome catalytic subunit, CSN5/Jab1. Echalier A, Pan Y, Birol M, Tavernier N, Pintard L, Hoh F, Ebel C, Galophe N, Claret FX, Dumas C; Proc Natl Acad Sci U S A. 2013;110:1273-1278. PMID:23288897

2: Crystal structure of the human COP9 signalosome. Lingaraju GM, Bunker RD, Cavadini S, Hess D, Hassiepen U, Renatus M, Fischer ES, Thoma NH; Nature. 2014;512:161-165. PMID:25043011