Proteasomal ubiquitin receptors

In eukaryotes, selective protein degradation is performed primarily by the ubiquitin-proteasome system (UPS). The 26S proteasome is a huge macromolecular complex composed of proteolytically active 20S core particle (CP) capped at one or both ends by a 19S regulatory particle (RP). The RP recognizes ubiquitinated substrates, deconjugates ubiquitin chains, and unfolds substrates before their translocation into the CP.
Until now, proteasome subunit Rpn10/S5a was shown to bind ubiquitin chains via ubiquitin-interaction motifs (UIMs). Additionally, shuttling receptors, that contain UBL and UBA domains, such as Rad23 (hHR23a/b in humans), Dsk2 (hPLIC-1/2 in humans) and Ddi1, deliver ubiquitinated cargo to the proteasome. Their UBA domains bind ubiquitin, while their UBL domains interact reversibly with the proteasome.
Using a yeast two-hybrid screen, with a bait of ubiquitin lacking the last two glycines to prevent its conjugation, we identified human Rpn13 (hRpn13, ADRM1 or ARM1) as ubiquitin-binding partner. Rpn13 is a part of 19S proteasome and its C-terminal region binds deubiquitinating enzyme Uch37/UCHL5 and enhances its isopeptidase activity. Rpn13 binds ubiquitin via a conserved N-terminal region termed the Pru domain (Pleckstrin-like receptor for ubiquitin), which binds K48-linked diubiquitin with an affinity of around 90 nM. Like proteasomal ubiquitin receptor Rpn10/S5a, Rpn13 also binds ubiquitin-like domains of the UBL/UBA family of ubiquitin receptors. A synthetic phenotype results in yeast when specific mutations of the ubiquitin binding sites of Rpn10 and Rpn13 are combined, indicating functional linkage between these ubiquitin receptors. Since Rpn13 is also the proteasomal receptor for Uch37, a deubiquitinating enzyme, our findings suggest a coupling of chain recognition and disassembly at the proteasome.
At the moment we are working on several novel Rpn13 interacting proteins, which should give us new intriguing clues about the function of Rpn13, and proteasome as a complex, in various processes in the cell.