This family was thought originally to be involved in cell-adhesion [1,2], but the members are now known to be proteasome subunit Rpn13, a novel ubiquitin receptor. The 26S proteasome is a huge macromolecular protein-degradation machine consisting of a proteolytically active 20S core, in the form of four disc-like proteins, and one or two 19S regulatory particles. The regulatory particle(s) sit on the top and or bottom of the core, de-ubiquitinate the substrate peptides, unfold them and guide them into the narrow channel through the centre of the core. Rpn13 and its homologues dock onto the regulatory particle through the N-terminal region which binds Rpn2. The C-terminal part of the domain binds de-ubiquitinating enzyme Uch37/UCHL5 and enhances its isopeptidase activity. Rpn13 binds ubiquitin via a conserved amino-terminal region called the pleckstrin-like receptor for ubiquitin, termed Pru, domain [4]. The domain forms two contiguous anti-parallel beta-sheets with a configuration similar to the pleckstrin-homology domain (PHD) fold [5]. Rpn13's ability to bind ubiquitin and the proteasome subunit Rpn2/S1 simultaneously supports evidence of its role as a ubiquitin receptor. Finally, when complexed to di-ubiquitin, via the Pru, and Uch37 via the C-terminal part, it frees up the distal ubiquitin for de-ubiquitination by the Uch37 [5].

1: Xoom: a novel oocyte membrane protein maternally expressed and involved in the gastrulation movement of Xenopus embryos. Hasegawa K, Shiraishi T, Kinoshita T; Int J Dev Biol 1999;43:479-485. PMID:10610020

2: Functional cloning of ARM-1, an adhesion-regulating molecule upregulated in metastatic tumor cells. Simins AB, Weighardt H, Weidner KM, Weidle UH, Holzmann B; Clin Exp Metastasis 1999;17:641-648. PMID:10919708

3: Cell biology: two hands for degradation. Saeki Y, Tanaka K; Nature. 2008;453:460-461. PMID:18497808

4: Proteasome subunit Rpn13 is a novel ubiquitin receptor. Husnjak K, Elsasser S, Zhang N, Chen X, Randles L, Shi Y, Hofmann K, Walters KJ, Finley D, Dikic I; Nature. 2008;453:481-488. PMID:18497817

5: Ubiquitin docking at the proteasome through a novel pleckstrin-homology domain interaction. Schreiner P, Chen X, Husnjak K, Randles L, Zhang N, Elsasser S, Finley D, Dikic I, Walters KJ, Groll M; Nature. 2008;453:548-552. PMID:18497827

6: A global census of fission yeast deubiquitinating enzyme localization and interaction networks reveals distinct compartmentalization profiles and overlapping functions in endocytosis and polarity. Kouranti I, McLean JR, Feoktistova A, Liang P, Johnson AE, Roberts-Galbraith RH, Gould KL; PLoS Biol. 2010; [Epub ahead of print] PMID:20838651