Degradation signals for ubiquitin system proteolysis in Saccharomyces cerevisiae

Abstract

Combinations of different ubiquitin-conjugating (Ubc) enzymes and other factors constitute subsidiary pathways of the ubiquitin system, each of which ubiquitinates a specific subset of proteins. There is evidence that certain sequence elements or structural motifs of target proteins are degradation signals which mark them for ubiquitination by a particular branch of the ubiquitin system and for subsequent degradation. Our aim was to devise a way of searching systematically for degradation signals and to determine to which ubiquitin system subpathways they direct the proteins. We have constructed two reporter gene libraries based on the lacZ or URA3 genes which, in Saccharomyces cerevisiae, express fusion proteins with a wide variety of C-terminal extensions. From these, we have isolated clones producing unstable fusion proteins which are stabilized in various ubc mutants. Among these are 10 clones whose products are stabilized in ubc6, ubc7 or ubc6ubc7 double mutants. The C-terminal extensions of these clones, which vary in length from 16 to 50 amino acid residues, are presumed to contain degradation signals channeling proteins for degradation via the UBC6 and/or UBC7 subpathways of the ubiquitin system. Some of these C-terminal tails share similar sequence motifs, and a feature common to almost all of these sequences is a highly hydrophobic region such as is usually located inside globular proteins or inserted into membranes.

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Endoplasmic reticulum stress-induced cell death mediated by the proteasome

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SCFCdc4-mediated Degradation of the Hac1p Transcription Factor Regulates the Unfolded Protein Response inSaccharomyces cerevisiae

“…This is the same E2 enzyme that we show below to mediate degradation of Hac1p. Because other degradation signals target a similar Ura3/HA fusion protein to different E2 enzymes such as Ubc6p and/or Ubc7p (Gilon et al, 1998), our data suggest that residues 122-158 of Hac1p contain a specific recognition signal for Ubc3p/Cdc34p. …”