Abstract: A protein reagent is provided for measuring protease enzyme activity in a sample. The protein reagent comprises two inhibition entities joined by a linker comprised of an indicator enzyme donor and an amino acid sequence susceptible to enzymatic cleavage. The protein reagent is substantially inhibited from binding to the cognate enzyme acceptor fragment, while the product of the enzymatic cleavage binds to the cognate enzyme acceptor fragment to form a functional indicator enzyme. The indicator enzyme activity is related to the protease enzyme activity of the sample.

Abstract: Mitosis of cells is determined, particularly in the presence of a candidate agent, using cells comprising members of an enzyme fragmentation complex pair, where one of the members is in the nucleus and the other member is in the cytoplasm. By growing the cells where mitosis may occur, one adds a substrate providing a detectable product, where the production of the detectable product is indicative of mitosis.

Abstract: Short enzyme donor fragments of ?-galactosidase are provided of not more than 40 amino acids, where the short fragments are used as a label and may be substituted with a wide variety of organic compounds, particularly polypeptides having independent functional activity. The enzyme donor finds use in competitive and non-competitive assays, monitoring intracellular events, or other processes where a sensitive non-interfering label is desired.

Abstract: Systems, including methods and reagents, for identifying enzyme inhibitors. The systems employ a conjugate of a known inhibitor of a target enzyme and an enzyme donor, an enzyme acceptor that binds to the enzyme donor to form an active indicator-enzyme complex, and a detectable substrate for the indicator enzyme. The assay is performed by combining the candidate agent, the conjugate of the known inhibitor and enzyme donor, the enzyme acceptor, and the substrate under binding conditions, where the candidate compound competes with the conjugate for the target enzyme. By measuring the rate of product formation or substrate depletion catalyzed by the indicator enzyme, the inhibitory activity of the candidate compound can be determined. The methodology is particularly applicable for target enzymes that have substrates or products that are difficult to synthesize and/or detect, such as kinases and phosphatases.