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Proceedings of the National Academy of Sciences of the United States of America logoLink to Proceedings of the National Academy of Sciences of the United States of America
. 1996 May 14;93(10):4907–4912. doi: 10.1073/pnas.93.10.4907

Using ubiquitin to follow the metabolic fate of a protein.

F Lévy 1, N Johnsson 1, T Rümenapf 1, A Varshavsky 1
PMCID: PMC39378  PMID: 8643502

Abstract

We describe a method that can be used to produce equimolar amounts of two or more specific proteins in a cell. In this approach, termed the ubiquitin/protein/reference (UPR) technique, a reference protein and a protein of interest are synthesized as a polyprotein separated by a ubiquitin moiety. This tripartite fusion is cleaved, cotranslationally or nearly so, by ubiquitin-specific processing proteases after the last residue of ubiquitin, producing equimolar amounts of the protein of interest and the reference protein bearing a C-terminal ubiquitin moiety. In applications such as pulse-chase analysis, the UPR technique can compensate for the scatter of immunoprecipitation yields, sample volumes, and other sources of sample-to-sample variation. In particular, this method allows a direct comparison of proteins' metabolic stabilities from the pulse data alone. We used UPR to examine the N-end rule (a relation between the in vivo half-life of a protein and the identity of its N-terminal residue) in L cells, a mouse cell line. The increased accuracy afforded by the UPR technique underscores insufficiency of the current "half-life" terminology, because in vivo degradation of many proteins deviates from first-order kinetics. We consider this problem and discuss other applications of UPR.

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Selected References

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