This family was thought originally to be involved in cell-adhesion, 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 homologs 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. 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.