The Saccharomyces cerevisiae ubiquitin-proteasome system

Our studies of the yeast ubiquitin-proteasome pathway have uncovered a numb er of general principles that govern substrate selectivity and proteolysis in this complex system. Much of the work has focused on the destruction of a yeast transcription factor, MAT alpha 2. The alpha 2 protein is polyubiqu itinated and rapidly degraded in alpha-haploid cells. One pathway of proteo lytic targeting, which depends on two distinct endoplasmic reticulum-locali zed ubiquitin-conjugating enzymes, recognizes the hydrophobic face of an am phipathic helix in alpha 2. Interestingly, degradation of alpha 2 is blocke d in a/alpha-diploid cells by heterodimer formation between the alpha 2 and al homeodomain proteins. The data suggest that degradation signals may ove rlap protein-protein interaction surfaces, allowing a straightforward steri c mechanism for regulated degradation. Analysis of alpha 2 degradation led to the identification of both 20S and 26S proteasome subunits, and several key features of proteasome assembly and active-site formation were subseque ntly uncovered. Finally, it has become clear that protein (poly)ubiquitinat ion is highly dynamic in vivo, and our studies of yeast de-ubiquitinating e nzymes illustrate how such enzymes can facilitate the proteolysis of divers e substrates.