A chemical-genetic approach for functional analysis of plant protein kinases

Plant genomes encode hundreds of protein kinases, yet only for a small fraction of them precise functions and phos-phorylation targets have been identified. Recently, we applied a chemical-genetic approach to sensitize the tomato serine/ threonine kinase Pto to analogs of PP1, an ATP-competitive and cell-permeable small-molecule inhibitor. The Pto kinase confers resistance to Pst bacteria by activating immune responses upon specific recognition of bacterial effectors. By using PP1 analogs in combination with the analog-sensitive Pto, we shed new light on the role of Pto kinase activity in effector recognition and signal transduction. Here we broaden the use of this chemical-genetic approach to another defense-related plant protein kinase, the MAP kinase LeMPK3. In addition, we show that analog-sensitive but not wild-type kinases are able to use unnatural N⁶-modified ATP analogs as phosphodonors that can be exploited for tagging direct phosphorylation targets of the kinase of interest. Thus, sensitization of kinases to analogs of the small-molecule inhibitor PP1 and ATP can be an effective tool for the discovery of cellular functions and phosphorylation substrates of plant protein kinases.