Cameleon is an engineered protein based on variant of green fluorescent protein

Green fluorescent protein

The green fluorescent protein is a protein composed of 238 amino acid residues that exhibits bright green fluorescence when exposed to blue light. Although many other marine organisms have similar green fluorescent proteins, GFP traditionally refers to the protein first isolated from the...

 used to visualize calcium

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 levels in living cells. It is a genetically encoded calcium sensor created by Roger Y. Tsien

Roger Y. Tsien

Roger Yonchien Tsien is a Chinese American biochemist and a professor at the Department of Chemistry and Biochemistry, University of California, San Diego...

 and coworkers. The name is a conflation of CaM (the common abbreviation of calmodulin

Calmodulin

Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...

) and chameleon

Chameleon

Chameleons are a distinctive and highly specialized clade of lizards. They are distinguished by their parrot-like zygodactylous feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, their swaying gait, the possession by many of a...

 to indicate the fact that the sensor protein undergoes a conformation change and radiates at an altered wavelength upon calcium binding to the calmodulin element of the Cameleon. Cameleon was the first genetically encoded calcium sensor that could be used for ratiometric measurements and has found many applications in neuroscience

Neuroscience

Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

 and other fields of biology. It was created by fusing BFP, calmodulin

Calmodulin

Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...

, calmodulin-binding peptide M13 and EGFP.

Mechanism

The DNA encoding cameleon fusion protein must be either stably or transiently introduced into cell of interest. Protein made by the cell according to this DNA information then serves as a fluorescent indicator of calcium concentration. In the presence of calcium, Ca²⁺ binds to M13, which enables calmodulin to wrap around the M13 domain. This brings the two GFP-variant proteins closer to each other, which increases FRET efficiency between them.