Photoprotein

They add to the function of the luciferins whose usual light-producing reaction is catalyzed by the enzyme luciferase.

The term photoprotein was first used to describe the unusual chemistry of the luminescent system of Chaetopterus (a marine Polychaete worm).

[1] This was meant to distinguish them from other light-producing proteins because these do not exhibit the usual luciferin-luciferase reaction.

Instead, when mixed with luciferin, they display luminescence proportional to the amount of the photoprotein.

Photoproteins form a stable luciferin-photoprotein complex, often until the addition of another required factor such as Ca2+ in the case of aequorin.