qMaLioffG: a genetically encoded green fluorescence lifetime-based indicator enabling quantitative imaging of intracellular ATP.

Genetically encoded indicators that can detect concentrations of metabolites and signalling molecules through fluorescence lifetime changes are gaining attention, because they expand the potential for quantitative imaging. These indicators offer advantages over conventional fluorescence intensity-based indicators by minimizing artifacts such as variations in indicator concentration, cellular morphological changes, and focus drift. However, the availability of fluorescence lifetime-based genetically encoded indicators remains limited, particularly those compatible with the widely used conventional 488 nm laser in microscopy. Here, we introduce qMaLioffG, a single green fluorescent protein-based ATP indicator that exhibits a substantial fluorescence lifetime shift (1.1 ns) within physiologically relevant ATP concentrations. This enables quantitative imaging of ATP levels in the cytoplasm and mitochondria under steady-state conditions across various cell types, providing insights into ATP distribution. We demonstrate that qMaLioffG can be used in multicellular systems, applying it to Drosophila brain and HeLa cell spheroids to reveal spatially heterogeneous ATP levels.