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Kinases are one of the most significant classes of drug targets because of their involvement in several diseases including cancer, osteoporosis, inflammatory disease, diabetes and neurodegeneration. Identification of candidate drugs targeting kinases relies on suitable bioassay techniques that can measure the activation by addition of phosphate group(s) to the protein or the inhibition of this reaction by compounds.
Historically kinase research has relied mostly on non-cell-based assays, however recent advances in reagent development have resulted in a more physiological method to measure candidate compound binding to a specific target kinase within intact cells. Below we describe the NanoBRET Target Engagement (TE) assay.
Promega’s NanoBRET™ TE kinase assay measures compound binding to full-length kinase targets in living cells. This quantitative analysis is based on Bio-Resonance Energy Transfer (BRET) between photons generated by a highly efficient luminescence enzyme, Nanoluc, and a cell permeable fluorophore tagged to a compound (tracer) that binds specifically to the kinase in living cells.
How it works
A cDNA construct consisting of the full-length kinase cDNA and Nanoluc luciferase enzyme cDNA are transiently expressed in cells (lipofection followed by overnight incubation at 37°C / 5% CO2) to generate a chimeric protein. Media is replaced with assay buffer containing the cell permeable Nanoluc substrate, generating photons that are detected in the fluorescence microplate reader at 460nm.
The tracer, which is a pan agonist labelled with a fluorophore, is added and binds to the kinase such that the fluorophore is brought into close proximity with the Nanoluc enzyme. This triggers the activation of the fluorophore via a process known as Bio Resonance Energy Transfer (BRET). The fluorophore emits at 610nm and this signal is measured within the fluorescence microplate reader.
The BRET response is a ratio of the signal at 610nm divided by the signal at 460nm. Both the tracer and the Nanoluc substrate are added simultaneously to the cells. The addition of test compounds results in competition for binding to the kinase, and the tracer is competed off the kinase, moving the fluorophore away from the 460nm photon source and thus decreasing the BRET signal at 610nm.
This process is a competition assay between the compound and the tracer for binding to the kinase allowing quantification of the intracellular affinity of the test compound within living cells at physiological pH and ATP concentration.