Cameleon is an engineered protein based on variant of green fluorescent protein used to visualize calcium levels in living cells. It is a genetically encoded calcium sensor created by Roger Y. Tsien and coworkers.[1] The name is a conflation of CaM (the common abbreviation of calmodulin) and chameleon 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 the first to be used in a transgenic animal to record activity in neurons and muscle cells.[2] Cameleon and other genetically encoded calcium indicators (GECIs) have found many applications in neuroscience and other fields of biology, including understanding the mechanisms of cell signaling by conducting time-resolved Ca2+ activity measurement experiments with endoplasmic reticulum (ER) enzymes.[3] It was created by fusing BFP, calmodulin, calmodulin-binding peptide M13 and EGFP.[4]

Mechanism

The DNA encoding cameleon fusion protein must be either stably or transiently introduced into the 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. A time-resolved spectroscopy study done on resonance energy transfer by Habuchi et al. in 2002 suggested the existence of 3 different calmodulin conformations that were dependent on Ca2+ binding. The study concluded that the mechanism of conformation interconversion remains unclear, but the data provided estimates of rate constants, energy transfer efficiency, and donor-acceptor distances in Ca2+-free and Ca2+-bound YC3.1 cameleon proteins.[5]

References

  1. ↑ Miyawaki A, Griesbeck O, Heim R, Tsien RY (1999). "Dynamic and quantitative Ca2+ measurements using improved Cameleons". Proc Natl Acad Sci USA. 96 (5): 2135–40. Bibcode:1999PNAS...96.2135M. doi:10.1073/pnas.96.5.2135. PMC 26749. PMID 10051607.
  2. ↑ Kerr R, Lev-Ram V, Baird G, Vincent P, Tsien RY, Schafer WR (2000). "Optical imaging of calcium transients in neurons and pharyngeal muscle of C. elegans". Neuron. 26 (3): 583–94. doi:10.1016/s0896-6273(00)81196-4. PMID 10896155.
  3. ↑ Demaurex, Nicolas; Frieden, Maud (2003-08-01). "Measurements of the free luminal ER Ca2+ concentration with targeted "cameleon" fluorescent proteins". Cell Calcium. 34 (2): 109–119. doi:10.1016/S0143-4160(03)00081-2. ISSN 0143-4160.
  4. ↑ Miyawaki A, Llopis J, Heim R, et al. (1997). "Fluorescent indicators for Ca2+ based on green fluorescent proteins and calmodulin". Nature. 388 (6645): 882–7. Bibcode:1997Natur.388..882M. doi:10.1038/42264. PMID 9278050.
  5. ↑ Habuchi, Satoshi; Cotlet, Mircea; Hofkens, Johan; Dirix, Gunter; Michiels, Jan; Vanderleyden, Jos; Subramaniam, Vinod; De Schryver, Frans C. (December 2002). "Resonance Energy Transfer in a Calcium Concentration-Dependent Cameleon Protein". Biophysical Journal. 83 (6): 3499–3506. doi:10.1016/s0006-3495(02)75349-6. ISSN 0006-3495. PMC 1302424. PMID 12496116.
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