Bright and stable: New acid-tolerant green fluorescent protein for bioimaging

Visualizing cellular components and processes at the molecular level is important for understanding the basis of any biological activity. Fluorescent proteins (FPs) are one of the most useful tools for investigating intracellular molecular dynamics.

However, FPs have usage limitations for imaging in low pH environments, such as in acidic organelles, including endosomes, lysosomes, and plant vacuoles. In environments of pH less than 6, most FPs lose their brightness and stability due to their neutral pK_a. pK_a is the measure of acid strength; the smaller the pK_a is, the more acidic the substance is.

“Although there are reports of several acid-tolerant green FPs (GFPs), most have serious drawbacks. Furthermore, there is a lack of acid-tolerant GFPs that are practically applicable to bioimaging,” says Hajime Shinoda, lead author of an Osaka University study that aimed to design acid-tolerant monomeric GFP that is practically applicable to live-cell imaging in acidic organelles. “In the current study, we developed an acid-tolerant GFP. We called it Gamillus.”

Gamillus is a GFP cloned from Olindias formosa (flower hat jellyfish) and exhibits superior acid tolerance (pK_a=3.4) and nearly twice as much brightness compared with the reported GFPs. The fluorescence spectrum is constant between pH4.5 and 9.0, which falls between the intracellular range in most cell types.

X-ray crystallography (a technique used for determining the atomic and molecular structure of a crystal, in this case, a Gamillus crystal) and point mutagenesis suggest the acid tolerance of Gamillus is attributed to stabilization of deprotonation in its chemical structure. The findings were published in Cell Chemical Biology.

“The applicability of Gamillus as a molecular tag was shown by the correct localization pattern of Gamillus fusions in a variety of cellular structures, including ones that are difficult to target,” corresponding author Takeharu Nagai says. “We believe Gamillus can be a powerful molecular tool for investigating unknown biological phenomena involving acidic organelles, such as autophagy.”