Structural Factors Enabling Successful GFP-Like Proteins with Alanine as the Third Chromophore-Forming Residue.

GFP-like proteins from lancelets (lanFPs) is a new and least studied group that already generated several outstanding biomarkers (mNeonGreen is the brightest FP to date) and has some unique features. Here, we report the study of four homologous lanFPs with GYG and GYA chromophores. Until recently, it was accepted that the third chromophore-forming residue in GFP-like proteins should be glycine, and efforts to replace it were in vain. Now, we have the first structure of a fluorescent protein with a successfully matured chromophore that has alanine as the third chromophore-forming residue. Consideration of the protein structures revealed two alternative routes of posttranslational transformation, resulting in either chromophore maturation or hydrolysis of GYG/GYA tripeptide. Both transformations are catalyzed by the same set of catalytic residues, Arg88 and Glu35-Wat-Glu211 cluster, whereas the residues in positions 62 and 102 shift the equilibrium between chromophore maturation and hydrolysis.