The reverse in-gel kinase assay to profile physiological kinase substrates

Elucidating kinase-substrate relationships is critical for understanding how phosphorylation affects signal transduction and regulatory cascades. Using the alpha catalytic subunit of protein kinase CK2 (CK2alpha) as a paradigm, we developed an in-gel method to facilitate identification of physiologic kinase substrates. In this approach, the roles of kinase and substrate in a classic in-gel kinase assay are reversed. In the reverse in-gel kinase assay (RIKA), a kinase is copolymerized in a denaturing polyacrylamide gel used to resolve a tissue or cell protein extract. Restoration of kinase activity and substrate structure followed by an in situ kinase reaction and mass spectrometric analyses results in identification of potential kinase substrates. We demonstrate that this method can be used to profile both known and novel human and mouse substrates of CK2alpha and cAMP-dependent protein kinase (PKA). Using widely available straightforward technology, the RIKA has the potential to facilitate discovery of physiologic kinase substrates in any biological system.