The green fluorescent protein (GFP) (Figure 1) is a protein found in the jellyfish (Aequorea Victoria). It is a very popular reporter gene, because it emits bright green light (λ = 509 nm) when excited with UV or blue light (λ max = 395 nm, minor 475 = nm). This makes it ideal for molecular biology because it can, for instance, tag other proteins.

Three D structure of GFP protein presented as the construction of many beta sheets shown as flat arrows connected by unstructured loops. Many beta sheets create barrel-like structure.

Figure 1. GFP structure.

In Aequorea Victoria, the protein responsible for bioluminescence capability is the Aequorin protein. This protein will emit blue light upon binding with Ca2+ ions. The blue light is then absorbed by GFP, which converts it into the green light.