The 26S proteasome is the major ATP-dependent protease in eukaryotes which plays a key role in intracellular protein degradation [1,2]. The lid of the 26S proteasome contains six PCI-domain-containing proteins (Rpn3/5/6/7/9/12), two MPN-domain-containing proteins (Rpn8/11), and one peptide, Sem1 [3]. The only catalytically active member of the lid is Rpn11, which serves as the essential deubiquitinase of the proteasome [3]. The C terminus of each subunit in the lid is predicted to form one or more helices. These C-terminal helices are highly conserved and have been predicted to form a helical bundle structure [3]. In yeast, Rpn9 was found to be necessary for the integrity and efficiency of the 26S proteasome [3,4]. This entry represents a helix domain C-terminal to the PCI domain found in Rpn9, a subunit of the 26S proteasome. C-terminal truncations of Rpn5 or Rpn9 have been shown not to cause any major lid assembly defects but to prevent the association of Rpn12 [3,4].

1: Dissection of the assembly pathway of the proteasome lid in Saccharomyces cerevisiae. Fukunaga K, Kudo T, Toh-e A, Tanaka K, Saeki Y; Biochem Biophys Res Commun. 2010;396:1048-1053. PMID:20471955

2: Structure of the human 26S proteasome at a resolution of 3.9 A. Schweitzer A, Aufderheide A, Rudack T, Beck F, Pfeifer G, Plitzko JM, Sakata E, Schulten K, Forster F, Baumeister W; Proc Natl Acad Sci U S A. 2016;113:7816-7821. PMID:27342858

3: Formation of an intricate helical bundle dictates the assembly of the 26S proteasome lid. Estrin E, Lopez-Blanco JR, Chacon P, Martin A; Structure. 2013;21:1624-1635. PMID:23911091

4: Solution structure of yeast Rpn9: insights into proteasome lid assembly. Hu Y, Wu Y, Li Q, Zhang W, Jin C; J Biol Chem. 2015;290:6878-6889. PMID:25631053