The C-terminal extension of the beta7 subunit and activator complexes stabilize nascent 20 S proteasomes and promote their maturation.

The eukaryotic 20 S proteasome is formed by dimerization of two precursor complexes containing the maturation factor Ump1. Beta7/Pre4 is the only one of the 14 subunits forming the 20 S proteasome that is absent from these precursor complexes in Saccharomyces cerevisiae. Increased expression of Pre4 leads to a reduction in the level of precursor complex, indicating that Pre4 incorporation into these complexes is rate-limiting for their dimerization.

Ubiquitin-dependent proteolytic control of SUMO conjugates.

Posttranslational protein modification with small ubiquitin-related modifier (SUMO) is an important regulatory mechanism implicated in many cellular processes, including several of biomedical relevance. We report that inhibition of the proteasome leads to accumulation of proteins that are simultaneously conjugated to both SUMO and ubiquitin in yeast and in human cells. A similar accumulation of such conjugates was detected in Saccharomyces cerevisiae ubc4 ubc5 cells as well as in mutants lacking two RING finger proteins, Ris1 and Hex3/Slx5-Slx8, that bind to SUMO as well as to the ubiquitin-conjugating enzyme Ubc4.

Assays for proteasome assembly and maturation.

The 20S proteasome is a complex multisubunit protease that is present in all phylae of life. Eukaryotic 26S proteasomes, which are composed of 20S proteasomes and 19S activator complexes, mediate the degradation of ubiquitylated proteins. Biogenesis of proteasomes involves a coordinated expression of proteasome genes as well as numerous assembly and maturation steps.