Green fluorescent protein, GFP, adopts a beta-barrel structure
with an alpha-helix inside. This structure is very rigid and the
chromophore is in the centre, protected by the protein.
Specific mutations in GFP residues can modulate its fluorescent
properties: different blue, cyan, green and yellow variants, have
been described. Using these proteins as starting point, different
calcium indicators have been developed by fusion of one or two GFP
derivatives, a calcium binding protein, such as calmodulin, CaM,
and/or a CaM binding peptide.
In general, those indicators can be classified depending on the
process that causes calcium signal.
Cameleons are based on a fluorescent
resonance energy transfer (FRET) process between two variants of
GFP. Camgaroos and pericams can
be classified as environment sensitive GFPs and there are also fusion
proteins (fusion aequorin-GFP and aequorin-RFP, red fluorescent
protein) that present chemiluminescence resonance energy transfer
(CRET or BRET) on binding calcium to aequorin.