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.
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.