In vitro reversible photoinactivation in a novel variant of Mnemiopsin 2

Incubation of Mnemiopsin with coelenterazine in a dark medium in the presence of oxygen molecules leads to the formation of functional bioluminescent complexes, which initiate light emission upon coordination of calcium ions. However, the functional complex is inhibited when exposed to environmental light. The photoinactivation is reversible in vivo by restoring the live organism to a dark medium, but it is irreversible in Mnemiopsin extracts in vitro. It has been suggested that the photoinactivation of Mnemiopsin results from the dissociation of coelenterazine and oxygen from the photoprotein. Accordingly, the dissociated chromophore differs from free coelenterazine due to the coordination of oxygen in its structure. In this study, while working on several mutants of Mnemiopsin 2, we accidentally observed that a mutant of Mnemiopsin 2, P181D, can recover its light-emitting ability after being treated with light. Compared with the wild-type Mnemiopsin, which completely loses its luminescence activity after 1 min of exposure to light, under similar conditions, the mutant exhibited 71 % of its original activity. Further studies showed that its activity after 60 min of exposure to light was 20.3 % of the original activity under standard conditions. To elucidate this observation, we extended our study and found that replacing Proline, a neutral residue with limited conformational space, with Aspartic acid, a charged residue with greater conformational space, increased the cooperativity of interactions within the photoprotein molecule and enhanced the affinity of the core structure for coelenterazine and oxygen.