The 26S proteasome is an ATP-dependent protease complex responsible for degrading many important cell regulators in plants and animals, especially those conjugated with multiple ubiquitins. It is composed of two subparticles, a 20S core protease (CP) that encloses the protease active sites and a 19S regulatory particle (RP) that binds to both ends of the CP and recruits appropriate substrates for degradation. Whereas the structure and proteolytic activities of the CP are understood at the atomic level, little is known about the structure of the RP and the functions of many of its principal subunits. It has also become clear in recent years that the currently defined RP actually represents the nucleus of an even more elaborate and dynamic complex that provides multiple routes for substrate recognition. These routes may be defined by specific isoforms of individual RP subunits, the association of the RP with a host of co-factors, the reversible binding of target shuttle proteins, and, in the most extreme cases, by the complete replacement of the RP with alternative subparticles. Genomic analyses suggest that plants in particular exploit this heterogeneity to generate a wide array of proteasome types, each with distinct compositions and unique functions/specificities. The goal of this project is to more fully define 26S proteasome function and diversity in plants by a more complete analysis of the particle from Arabidopsis thaliana.