Exploiting protein destruction for constructive use

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Abstract
Discovering the biological roles of a protein is best accomplished by observing the consequences of its removal. However, such loss-of-function studies are rarely straightforward. In the case of genetic experiments, including those dealing with knockout mice, gene functions in one tissue type are often disguised by deleterious phenotypes, including lethality, in another. Even when this problem is overcome, for example, by tissue-specific gene deletion using the Cre/loxP system, it can be difficult to dissect primary from secondary effects to determine the molecular basis of a phenotype. The difficulty lies in the speed at which the biological events being studied occur. For example, after transcription of the Cre recombinase is induced, considerable time will lapse before recombination of the targeted gene and dissipation of preexisting pools of the target gene's RNA and protein. During this period, the studied cells could have, for example, responded to extracellular signals, undergone cell divisions, changed position or shape, and even differentiated into a new cell type. Alternative methods, such as RNA interference or small-molecule inhibition, allow regulation of the protein of interest during tighter time windows. Unfortunately, these techniques have their own shortcomings. RNA interference suffers from nonspecific effects, unpredictable degrees of “knockdown,” and slow kinetics of onset and reversibility. Small-molecule regulation is generally very fast and usually reversible; however, identifying or developing a small molecule that is genuinely specific with reliable pharmacokinetics challenges even the largest pharmaceutical company. To this end, researchers have devoted considerable energy to develop new technologies that merge gene-based methods (to create impeccable specificity) with chemical-based strategies (to provide rapid on/off regulation). In a recent issue of PNAS, Pratt et al. (1) report a new approach that uses a generic drug to induce the recovery of a native target protein from a fusion protein that is otherwise destined for destruction …