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Cell. 1998 Jul 24;94(2):217-27.

Degradation signal masking by heterodimerization of MATalpha2 and MATa1 blocks their mutual destruction by the ubiquitin-proteasome pathway.

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Department of Biochemistry and Molecular Biology, University of Chicago, Illinois 60637, USA.


Proteolysis by the ubiquitin-proteasome pathway is often regulated, but the mechanisms underlying such regulation remain ill-defined. In Saccharomyces cerevisiae, cell type is controlled by the MAT transcription factors. The alpha2 repressor is a known ubiquitin pathway substrate in alpha haploid cells. We show that a1 is rapidly degraded in a haploids. In a/alpha diploids, alpha2 and a1 are stabilized by heterodimerization. Association depends on N-terminal coiled-coil interactions between a1 and alpha2. Residues in alpha2 important for these interactions overlap a critical determinant of an alpha2 degradation signal, which we delimit by extensive mutagenesis. Our data provide a detailed description of a natural ubiquitin-dependent degradation signal and point to a molecular mechanism for regulated turnover in which proteolytic signals are differentially masked in alternative multiprotein complexes.

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