|M.Sc Thesis||Department of Biology|
|Supervisors:||Prof. Glickman Michael|
|Dr. Elah Pick|
The 26S proteasome is a 2.5-Mega Dalton protein degradation machine built of over 33 diverse components. 14 of these subunits form a 20S catalytic core particle (CP) responsible for degradation of protein substrates, and the remainder create the 19S regulatory particle (RP) capping the 20S CP on one or both sides to regulate proteolysis. The 19S RP can be further dissociated into two distinct structures known as the "lid" and the "base".
The base attaches directly to the 20S CP surface and contains a ring of six homologous ATPase subunits of the AAA family (referred to as Rpt1-6), plus Rpn1 and Rpn2, two non-ATPases. The base interfaces directly with the outer surface of the CP. The base carries out unfolding of substrates prior to the degradation in the CP. Two other components of the base are the ubiquitin receptors Rpn10 and Rpn13 that recognize ubiquitin tags on proteasome substrates. The lid is composed of over eight essential non-ATPase subunits (Rpn3, Rpn5-9, Rpn11-12). Homologues of lid subunits are found in two other cellular complexes, the COP9 Signalosome (CSN) and the eukaryotic translation factor eIF3. The canonical version of these complexes comprising of six subunits bearing a PCI domain alongside two subunits bearing an MPN domain is found in all multi-cellular organisms. Among the PCI complexes, the proteasome lid and the CSN are particularly analogous with similar architecture and one-to-one subunit paralogy. Functional CSN complexes exist in single-celled organisms. However, fewer CSN subunits are identified. The budding yeast S. cerevisiae contains a divergent "CSN-like" complex, with only one clear ortholog (CSN5/Rri1), and up to five distant orthologs. eIF3 also diverges greatly. Interestingly, one subunit may be shared, Csn11/Pci8, was found associated to eIF3 and CSN, potentially explaining the seemingly lower number of dedicated subunits in this organism. A similar situation may exist vis-à-vis the lid: several CSN subunits interact with Rpn5, a component of the proteasome lid. Possibly Rpn5 may also double-up as a component of two PCI complexes.
In this study, we charted the associations of Rpn5 with the CSN and characterized the effect of rpn5 loss-of-function on CSN-related phenotypes. We found that in budding yeast this proteasome lid subunit is also a component of the CSN and required for cullin derubylation. Participation in two related complexes sheds light on the evolutionary origin of PCI-containing proteins.