Methods for measuring protein kinase and phosphatase activity

The invention provides fluorescence-based assays for determining the phosphorylating or the dephosphorylating activity of an enzyme acting on a substrate molecule. The substrate comprises at least one moiety that is capable of being phosphorylated or dephosphorylated by an enzyme to yield a product, the substrate being labelled with a fluorescent dye, and which is capable of exhibiting change in fluorescence intensity and fluorescence lifetime upon phosphorylation of the dye-labelled substrate or upon dephosphorylation of the dye-labelled phosphorylated substrate. In preferred embodiments, the substrate is labelled with a fluorescent dye selected from the acridone and the quinacridone classes of dyes. Also provided is a method is provided for screening for a test agent whose effect on the phosphorylating or dephosphorylating activity of an enzyme is to be determined.