The PRU 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. Rpn13 binds ubiquitin via a conserved amino-terminal region called the pleckstrin-like receptor for ubiquitin, termed Pru, domain (3, 4). The domain forms two contiguous anti-parallel beta-sheets with a configuration similar to the pleckstrin-homology domain (PHD) fold. 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. Hasegawa et al., 1999. Xoom: a novel oocyte membrane protein maternally expressed and involved in the gastrulation movement of Xenopus embryos. Int J Dev Biol., 43(6):479-85. PMID: 10610020

2. Simins et al., 1999. Functional cloning of ARM-1, an adhesion-regulating molecule upregulated in metastatic tumor cells. Clin Exp Metastasis., 17(8):641-8. PMID: 10919708

3. Husnjak et al., 2008. Proteasome subunit Rpn13 is a novel ubiquitin receptor. Nature., 453(7194):481-8. PMID: 18497817

4. http://www.ebi.ac.uk/interpro/entry/IPR006773

5. Schreiner et al., 2008. Ubiquitin docking at the proteasome through a novel pleckstrin-homology domain interaction. Nature., ;453(7194):548-52. PMID: 18497827