Ca²⁺-triggered coelenterazine-binding protein (CBP) is a natural form of the luciferase substrate involved in the Renilla bioluminescence reaction. It is a stable complex of coelenterazine and apoprotein that, unlike coelenterazine, is soluble and stable in an aquatic environment and yields a significantly higher bioluminescent signal. This makes CBP a convenient substrate for luciferase-based in vitro assay. In search of a similar substrate form for the luciferase NanoLuc, a furimazine-apoCBP complex was prepared and verified against furimazine, coelenterazine, and CBP. Furimazine-apoCBP is relatively stable in solution and in a frozen or lyophilized state, but as distinct from CBP, its bioluminescence reaction with NanoLuc is independent of Ca²⁺. NanoLuc turned out to utilize all the four substrates under consideration. The pairs of CBP-NanoLuc and coelenterazine-NanoLuc generate bioluminescence with close efficiency. As for furimazine-apoCBP-NanoLuc pair, the efficiency with which it generates bioluminescence is almost twice lower than that of the furimazine-NanoLuc. The integral signal of the CBP-NanoLuc pair is only 22% lower than that of furimazine-NanoLuc. Thus, along with furimazine as the most effective NanoLuc substrate, CBP can also be recommended as a substrate for in vitro analytical application in view of its water solubility, stability, and Ca²⁺-triggering “character”.

Currently, adeno-associated virus (AAV) is one of the most reliable carrier for gene delivery in both proliferating and non-proliferating cells. Stable and long-lasting transgene expression has made this viral vector a key platform for the development of advanced therapy. Nevertheless, the widespread clinical use of AAV-based drugs remains limited due to their immunogenicity, low capsid capacity, and restricted tissue tropism. Tissue tropism depends largely on the the transduction efficiency of AAV capsids. In this study, we modified the standard three-plasmid transfection protocol to provide independent expression of VP1 or VP2 proteins from separate plasmids. Adjusting the ratio of these plasmids in the transfection mixture enabled alteration of the stoichiometric composition of the capsids, as SDS-PAGE and mass spectrometry confirmed. Increasing the amount of VP1 or VP2 in the capsid composition enhanced transduction efficiency, as demonstrated in vitro experiments on HEK293 cells. Obtained results contribute to a more comprehensive understanding of the AAV biology and have perspective of application in gene therapy.