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J Biol Chem. 1996 Oct 18;271(42):25790-6.

A yeast Ubc9 mutant protein with temperature-sensitive in vivo function is subject to conditional proteolysis by a ubiquitin- and proteasome-dependent pathway.

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Institut für Genetik und Mikrobiologie, Universität München, Maria-Ward-Strasse 1a, D-80638 München, Germany.


The UBC9 gene of the yeast Saccharomyces cerevisiae is essential for cell viability and encodes a soluble protein of the nucleus that is metabolically stable. Products of mutant alleles selected to confer temperature-sensitive in vivo function were found to be extremely short-lived at the restrictive but long-lived at the permissive condition. An extragenic suppressor mutation was isolated which increased thermoresistance of a ubc9-1 strain. This suppressor turned out to stabilize the mutated gene product, indicating that the physiological activity of ubc9-1 protein is primarily controlled by conditional proteolysis. The labile ubc9-1 protein appears to be a substrate for ubiquitination, and its turnover was substantially reduced by expression of a ubiquitin derivative that interferes with formation of multi-ubiquitin chains. Stabilization resulted also from competitive inhibition of Ubc4-related ubiquitin-conjugating enzymes. Activity of the proteasome complex was crucial to rapid breakdown, whereas vacuolar proteases were dispensable. Thus, the heat-denatured ubc9-1 protein is targeted for proteolysis by the ubiquitin-proteasome pathway and may serve as a useful tool to further define the process by which a misfolded polypeptide is recognized.

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