A BRET-based homogeneous insulin assay using interacting domains in the primary binding site of the insulin receptor

A new homogeneous insulin assay requiring no chemical modification of an insulin recognition domain, which can be applied to continuous monitoring of the time-dependent cellular response in vitro, was developed. The carboxy-terminal α-chain (αCT) segment and first leucine-rich-repeat (L1) domain in the primary binding site on the insulin receptor were genetically fused with a bioluminescent protein (Nanoluc, Nluc) and a fluorescent protein (yellow fluorescent protein, YPet) to produce the insulin-sensing probe proteins Nluc-αCT and L1-YPet. The BRET signal was observed on simple mixing of insulin with these protein probes, in a so-called homogeneous assay. The BRET signal was proportional to the insulin concentration, and the lower detection limit was 0.8 μM. Time-dependent insulin secretion from drug-stimulated MIN6 cells was also successfully monitored continuously with the probe proteins. This BRET-based homogeneous insulin assay method is thus expected to be applicable to drug development by high-throughput screening.