A monochromatically excitable green-red dual-fluorophore fusion incorporating a new large Stokes shift fluorescent protein
Genetically encoded sensors enable quantitative imaging of analytes in live cells. State-of-the-art sensors are commonly constructed by combining ligand-binding domains with one or more sensitized fluorescent protein (FP) domains. Sensors based on a single FP are susceptible to artifacts caused by differing expression levels or sensor distribution in vivo. Hence, our lab developed dual-FP Matryoshka technology introduced by a single cassette that contains a stable large Stokes shift (LSS) reference FP nested within a reporter FP (cpEGFP), allowing simple construction of intensiometric sensors with the capacity for ratiometric quantification. The first-generation Green-Orange (GO) Matryoshka cassette established proof of concept but required custom optical setups to maximize achievable dynamic range. Here, we present a genetically encoded calcium sensor that employs optimized second-generation Green-Apple (GA) Matryoshka technology that incorporates a newly designed red LSSmApple fluorophore. LSSmApple provides improved excitation spectrum overlap with cpEGFP, allowing for monochromatic co-excitation with blue light. The exceptionally large Stokes shift of LSSmApple results in improved emission spectrum separation from cpEGFP, which minimizes fluorophore bleed-through and facilitates imaging using standard dichroics and red fluorescent protein (RFP) emission filters. We developed an image analysis pipeline for yeast (Saccharomyces cerevisiae) timelapse imaging that utilizes LSSmApple to segment and track cells for high-throughput quantitative analysis. In summary, we engineered a new fluorescent protein, constructed a genetically encoded calcium indicator (GA-MatryoshCaMP6s), and performed calcium imaging in yeast as a demonstration.