In addition to manipulating upstream receptors and downstream molecules in kinase signaling cascades, another approach involves identifying direct kinase substrates in the hope that some will be useful targets for pharmaceutical development. Unfortunately, for most kinases this has been a difficult task and progress has been limited. Traditionally, kinase substrates have been identified pharmacologically using kinase activators or inhibitors. The specificity of these drugs can be problematic and the results of such studies often fail to identify a kinase substrate.282 However, Shokat and colleagues283 have promoted a novel chemical-genetics approach to identify kinase substrates that target the structurally conserved purine-binding pocket found in all kinases. The strategy involves generating mutant alleles that can use ATP analogs in addition to ATP to phosphorylate substrate proteins. The mutation creates a “hole” in the pocket by replacing a conserved bulky amino acid with a smaller residue (Fig. 2). ATP analogs (*ATP) possessing a side group “bump” designed to fit into the hole can be used to identify substrates specific to the analog-sensitive (as) kinase mutant. Specificity for the mutant kinase is achieved because
