What activates GFP?

When GFP is exposed to light in the blue to ultraviolet range, it emits green fluorescence. GFP has 238 amino acids, in which three of them (amino acids 65-67) creates a structure that emits visible green fluorescent light. In the jellyfish, GFP interacts with the protein aequorin which emits blue light when added with calcium. More specifically, three calcium ions become bound to the aequorin, which oxidizes the coelenterazine with a protein-bound oxygen, creating a Ca3-apo-aequorin-coelenteramide complex. This complex emits blue light in vitro. Then, in vivo, the aequorin complex goes through radiationless energy transfer to GFP, where GFP absorbs the blue light. This gives it its green fluorescence color. It is important to note that GFP light is excited at the 395 nm wavelength and optimally detected at 509 nm.