The short-lived Mat alpha 2 transcriptional repressor is protected from degradation in vivo by interactions with its corepressors Tup1 and Ssn6

The Mat alpha 2 (alpha 2) protein is a transcriptional repressor necessary for the proper expression of cell type-specific genes in Saccharomyces cerevisiae. Like many transcription factors, alpha 2 is rapidly degraded in vivo by the ubiquitin-proteasome pathway. At least two different ubiquitin-dependent pathways target alpha 2 for destruction, one of which recognizes the well-characterized Deg1 degradation determinant near the N terminus of the protein. Here we report that the alpha 2 corepressors Tup1 and Ssn6 modify the in vivo degradation rate of alpha 2. Tup1 modulates the metabolic stability of alpha 2 by directly binding to the Deg1-containing region of the protein. TUP1 overexpression specifically stabilizes Deg1-containing proteins but not other substrates of the same ubiquitination enzymes that recognize Deg1. Point mutations in both alpha 2 and Tup1 that compromise the alpha 2-Tup1 binding interaction disrupt the ability of Tup1 to stabilize Deg1 proteins. The physical association between Tup1 and alpha 2 competes with the ubiquitination machinery for access to the Deg1 signal. Finally, we observe that overproduction of both Tup1 and Ssn6, but not either alone, strongly stabilizes the endogenous alpha 2 protein. From these results, we propose that the fraction of alpha 2 found in active regulatory complexes with Tup1 and Ssn6 is spared from rapid proteolytic destruction and is stabilized relative to the uncomplexed pool of the protein.