Acid-Tolerant Monomeric GFP from Olindias formosa

The fluorescent protein (FP) color palette has greatly contributed to the visualization of molecular and cellular processes. However, most FPs lose fluorescence at a pH lower than their neutral pK_a (∼6), and this has hampered their application in acidic organelles (pH ∼4.5-6.0). Currently, several cyan- and red-colored acid-tolerant FPs are available; however, there are few reports of acid-tolerant green FPs (GFPs) that are practically applicable to bioimaging. Here, we developed the acid-tolerant monomeric GFP "Gamillus" from the jellyfish Olindias formosa, with excellent brightness, maturation speed, and photostability. Results from X-ray crystallography and point mutagenesis suggest that across a broad pH range the acid tolerance is attributed to stabilization of deprotonation in the chromophore phenyl ring by forming a unique trans configuration. We demonstrate that Gamillus can serve as a molecular tag suitable for imaging in acidic organelles through autophagy-mediated molecular tracking to lysosomes.