DEGRADATION SIGNALS FOR UBIQUITIN SYSTEM PROTEOLYSIS IN SACCHAROMYCES-CEREVISIAE

Combinations of different ubiquitin-conjugating (Ubc) enzymes and othe r factors constitute subsidiary pathways of the ubiquitin system, each of which ubiquitinates a specific subset of proteins. There is eviden ce that certain sequence elements or structural motifs of target prote ins are degradation signals which mark them for ubiquitination by a pa rticular branch of the ubiquitin system and for subsequent degradation . Our aim was to devise a way of searching systematically for degradat ion signals and to determine to which ubiquitin system subpathways the y direct the proteins. We have constructed two reporter gene libraries based on the lacZ or URA3 genes which, in Saccharomyces cerevisiae, e xpress fusion proteins with a wide variety of C-terminal extensions. F rom 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 mutant s. The C-terminal extensions of these clones, which vary in length fro m 16 to 50 amino acid residues, are presumed to contain degradation si gnals channeling proteins for degradation via the UBC6 and/or UBC7 sub pathways 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 in side globular proteins or inserted into membranes.