The amino acid sequences of ten proteins with intracellular half-lives less than 2 hours contain one or more regions rich in proline (P), glutamic acid (E), serine (S), and threonine (T). These PEST regions are generally, but not always, flanked by clusters containing several positively charged amino acids. Similar inspection of 35 proteins with intracellular half-lives between 20 and 220 hours revealed that only three contain a PEST region. On the basis of this information, it was anticipated that caseins, which contain several PEST sequences, would be rapidly degraded within eukaryotic cells. This expectation was confirmed by red blood cell-mediated microinjection of 125I-labeled caseins into HeLa cells where they exhibited half-lives of less than 2 hours. The rapid degradation of injected alpha- and beta-casein as well as the inverse correlation of PEST regions with intracellular stability indicate that the presence of these regions can result in the rapid intracellular degradation of the proteins containing them.

In 1986, we proposed that polypeptide sequences enriched in proline (P), glutamic acid (E), serine (S) and threonine (T) target proteins for rapid destruction. For much of the past decade there were only sporadic experimental tests of the hypothesis. This situation changed markedly during the past two years with a number of papers providing strong evidence that PEST regions do, in fact, serve as proteolytic signals. Here, we briefly review the properties of PEST regions and some interesting examples of the conditional nature of such signals. Most of the article, however, focuses on recent experimental support for the hypothesis and on mechanisms responsible for the rapid degradation of proteins that contain PEST regions.

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We have identified a novel 200 kDa nuclear protein that activates the proteasome. The protein, which we call PA200, has been purified to homogeneity from bovine testis and has been shown to activate proteasomal hydrolysis of peptides, but not proteins. Following γ‐irradiation of HeLa cells the uniform nuclear distribution of PA200 changes to a strikingly punctate pattern, a behavior characteristic of many DNA repair proteins. Homologs of PA200 are present in worms, plants and yeast. Others have shown that mutation of yeast PA200 results in hypersensitivity to bleomycin, and exposure of yeast to DNA damaging agents induces the PA200 message. Taken together, these findings implicate PA200 in DNA repair, possibly by recruiting proteasomes to double strand breaks.