Abstract
Protein kinases are enzymes that regulate many cellular events in eukaryotic cells, such as cell-cycle progression, transcription, metabolism, and apoptosis. Protein kinases each have a conserved ATP-binding site, as well as one or more substrate-binding site(s) that exhibit recognition features for a protein substrate. Thus, by bringing ATP and a substrate into close proximity, each protein kinase can modify its substrate by transferring the gamma phosphate of the ATP molecule to a serine, threonine, or tyrosine residue on the substrate. In such a way, signaling pathways downstream from the substrate can be regulated, dependent on the phosphorylated versus dephosphorylated forms of the substrate. This unit describes an assay employing a fluorescent peptide substrate to measure the incorporation of non-radiolabeled phosphate. The assay is based on the principle that the phosphorylation of the peptide substrate leads to an increase in the fluorescence emission intensity of an appended fluorophore.