Application of a novel HiBiT peptide tag for monitoring ATF4 protein expression in Neuro2a cells

A split NanoLuc assay system consisting of two fragments, large N-terminal and small C-terminal regions (NanoBiT), was developed to investigate protein-protein interactions within living cells. Interestingly, the replacement of five amino acids among 11 C-terminal amino acids dramatically increased affinity against the large N-terminal fragment, LgBiT, and the complex had NanoLuc luciferase activity. In this study, we first applied this small fragment, HiBiT, to elucidate the expression of ATF4 protein by transient overexpression of HiBiT-tagged ATF4. According to the regulation of intrinsic ATF4 protein, stabilization of HiBiT-tagged ATF4 with a proteasome inhibitor, MG132, was observed by detecting luciferase activity in cell lysate and after SDS-PAGE and transfer onto a PVDF membrane. Next, we knocked-in the HiBiT-epitope tag into the ATF4 gene using the CRISPR/Cas9 system and rapidly selected positive clones by measuring luciferase activity in an aliquot of each cell suspension. Using a selected clone, we observed that the expression of HiBiT-tagged ATF4 in the selected cells varied in response to treatment with protein synthesis inhibitors or proteasome inhibitors and tunicamycin. Altogether, this novel HiBiT tag is a useful tool to evaluate the endogenous expression levels of proteins of interest.