Now that the genome of certain organisms has been revealed, some scientists are turning their attention to questions posed as a result of the information, spawning the emergence of new scientific disciplines. For example, scientists are now working to characterize all the proteins, the "master parts list," encoded by the genetic blueprint. Researchers are particularly interested in determining how various proteins interact to direct the activities within a cell, or even neighboring cells.
Proteomics is the study of the proteome, the inventory of proteins encoded by a genome. One important aspect of proteomics is to discover which of the many proteins in a cell interact with each other. A powerful tool for studying protein-protein interactions is the yeast two-hybrid system. This ingenious technique relies on two yeast proteins that work in concert to activate a reporter gene; one binds to a specific sequence of DNA and the other activates the gene. The two proteins can do this, however, only when they are physically linked. The yeast two-hybrid system involves two separate preparations. First, a specialized cDNA library of the organism of interest is created, giving rise to fusions
between the DNA-binding protein of the yeast two-hybrid system and the various proteins encoded by the cloned genes. Then, another recombinant molecule is made; this one creates a fusion between the activating protein of the yeast two-hybrid system and the protein of interest. This second recombinant molecule will function as a "lure" that can be used to "fish" for proteins in the cDNA library that interact with the protein of interest. When the "lure" is transferred into cells that harbor the cDNA library, the reporter gene will be activated in those cells that express a protein that interacts with the protein of interest. The interactions of various proteins in the yeast Saccharomyces cerevisiae have already been characterized using the yeast two-hybrid system. Currently, researchers are attempting to map the protein-protein interactions of the entire human proteome. The fact that cells modify many proteins after they are made makes this project much more challenging than it might seem.
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