This domain is found at the N-terminus of the 26S proteasome regulatory subunits RPN1 (also known as 26S proteasome non-ATPase regulatory subunit 2 (PMSD2)[1]. The domain is formed by an array of alpha helices [2].

1: The life cycle of the 26S proteasome: from birth, through regulation and function, and onto its death. Livneh I, Cohen-Kaplan V, Cohen-Rosenzweig C, Avni N, Ciechanover A; Cell Res. 2016;26:869-885. PMID:27444871

2: 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