The short-lived matα2 transcriptional repressor is protected from degradation in vivo by interactions with its compressors Tup1 and Ssn6

The Matα2 (α2) protein is a transcriptional repressor necessary for the proper expression of cell type-specific genes in Saccharomyces cerevisiae. Like many transcription factors, α2 is rapidly degraded in vivo by the ubiquitin-proteasome pathway. At least two different ubiquitin-dependent pathways target α2 for destruction, one of which recognizes the well-characterized Deg1 degradation determinant near the N terminus of the protein. Here we report that the α2 corepressors Tup1 and Ssn6 modify the in vivo degradation rate of α2. Tup1 modulates the metabolic stability of α2 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 α2 and Tup1 that compromise the α2-Tup1 binding interaction disrupt the ability of Tup1 to stabilize Deg1 proteins. The physical association between Tup1 and α2 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 α2 protein. From these results, we propose that the fraction of α2 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.