Compositions, methods, and kits for detecting and monitoring post-translational modification activities, including kinase or phosphatase activities, are described. The compositions typically include a peptide, a first detectable moiety, a first binding member, and a protease cleavage site. Modification of a composition by a post-translational modification enzyme, such as a kinase or phosphatase, alters the proteolytic sensitivity of the peptide, resulting in a change of a detectable property of the composition when it is associated noncovalently with a probe composition that includes a second binding member and a second detectable moiety. Panel assays for determining substrates or modulators of enzymatic activities are also described.