Compositions, methods, and kits for detecting and monitoring kinase, phosphatase and protein post-translational modification activity are described. The compositions typically include a peptide, a detectable moiety, and a protease cleavage site. Modification of a peptide by a kinase, phosphatase or other protein post-translational modification alters the proteolytic sensitivity of the peptide, resulting in a change of a detectable property of the composition. Panel assays for determining substrates or modulators of kinase, phosphatase or other protein post-translational modification activity are also described.