Different categories of fluorescent proteins result in GEVIs with similar characteristics

The latest generation of genetically encoded voltage indicators (GEVIs) is significantly advancing our ability to study electrical activity from large numbers of identified neurons. The further refinement of the technology will contribute to our understanding of behavior-evoked information perception, transfer and processing on a cellular level across brain regions. The development of GEVIs relies on synthetic biology which includes rational and random modifications of indicator sequence. One strategy in GEVI design is based on creating chimeras between voltage sensitive protein domains (VSDs) and fluorescent proteins (FPs). However, in this design scenario, the mechanistic details of voltage-induced fluorescence change that would inform rational design and improvements of GEVIs are still largely missing. Here we preformed a systematic study of how nature of the FP and altering the insertion site affects the characteristics of Ciona intestinalis voltage-sensitive phosphatase-based GEVIs. Surprisingly, we found that regardless of vast difference in phylogenesis, biochemical properties, fluorophore structure, sequence and excitation/emission spectra between FPs, the resulting GEVIs exhibit virtually identical decrease in fluorescence intensity in response to depolarization. These results stand in strong contrast to studies demonstrating that small numbers of targeted mutations in the FP sequence cause dramatic changes in both signal size and polarity.