This is the C-terminal domain found in S. cerevisiae Rpn2 (26S proteasome regulatory subunit RPN2) as well as other eukaryotic species. A study revealed that the C-terminal 52 residues of the Rpn2 C-terminal domain are responsible for mediating interactions with the ubiquitin-binding subunit Rpn13. Futhermore, the extreme C-terminal 20 or 21 residues of Rpn2 (926-945 or 925-945) of S. cerevisiae, were shown to be equally effective at binding Rpn13. Multiple sequence alignments indicate that Rpn2 orthologs are highly conserved in this C-terminal region and share characteristic acidic, aromatic, and proline residues, suggesting a common function. In the structure of Rpn2 from S. cerevisiae , this region is exposed and disordered, and is thus accessible for associating with Rpn13. The Rpn2 binding surface of human Rpn13 has been mapped by nuclear magnetic resonance titration to one surface of its Pru domain [1].

1: The structure of the 26S proteasome subunit Rpn2 reveals its PC repeat domain as a closed toroid of two concentric alpha-helical rings. He J, Kulkarni K, da Fonseca PC, Krutauz D, Glickman MH, Barford D, Morris EP; Structure. 2012;20:513-521. PMID:22405010

2: Deep classification of a large cryo-EM dataset defines the conformational landscape of the 26S proteasome. Unverdorben P, Beck F, Sledz P, Schweitzer A, Pfeifer G, Plitzko JM, Baumeister W, Forster F; Proc Natl Acad Sci U S A. 2014;111:5544-5549. PMID:24706844