aequorin

Aequorin is a calcium-activated photoprotein isolated from the jellyfish Aequorea victoria. It emits blue light when bound to the luciferin coelenterazine in the presence of calcium ions (Ca2+), making it a widely used bioluminescent reporter for monitoring intracellular calcium dynamics.

Structurally, aequorin is a ~22 kDa protein that contains a prosthetic group formed by noncovalently bound coelenterazine. The protein has calcium-binding motifs that sense intracellular Ca2+ levels. Upon Ca2+ binding, coelenterazine is oxidized and decarboxylated to coelenteramide, releasing energy that excites the oxidized luciferin and produces blue light with a peak near 469 nm.

Aequorin was first purified and characterized by Osamu Shimomura in the 1960s from Aequorea victoria. Its discovery enabled the development of calcium imaging methods before the advent of fluorescent calcium indicators. Recombinant aequorin has facilitated intracellular calcium measurements in various cell types, often by delivering the apo protein and then supplying coelenterazine to reconstitute active aequorin in situ.

Applications and limitations: Aequorin-based luminescence provides a light-based readout of Ca2+ transients without external illumination, reducing photodamage in some experiments. It is powerful for rapid, temporal measurements of calcium dynamics. Limitations include dependence on the substrate coelenterazine, non-ratiometric signal that can be influenced by protein expression levels, and a decay in signal as intracellular Ca2+ is buffered and coelenterazine is depleted. Variants and chimeric approaches have extended its use, including combinations with fluorescent reporters for complementary readouts.