Subunit Organization of the 26S Proteasome and Structural Basis for Processing of Ubiquitin-Tagged Substrates

Abstract

In eukaryotic cells, the proteasome degrades unwanted proteins by recognizing specific polyubiquitin tags covalently attached to these proteins. The precise manner in which these ubiquitin chains are recognized and removed from the targeted proteins prior to proteolysis is poorly understood. This is partly due to a lack of structural information on the ubiquitin-recognizing components of the proteasome 19S regulatory particle. Using a recombinant expression system and electron microscopy, we were able to localize all subunits of the yeast 19S particle, and elucidate the spatial arrangement of ubiquitin receptors, deubiquitinating enzymes, and the protein unfolding machinery. Our studies also revealed large conformational rearrangements in the lid subcomplex upon holoenzyme formation, suggesting an allosteric mechanism for activation of its deubiquitination activity. From these studies, we have a much better understanding of the manner in which the 26S recognizes and deubiquitinates proteins marked for proteolysis.