A unique EF-hand motif in mnemiopsin photoprotein from Mnemiopsis leidyi: Implication for its low calcium sensitivity

► The first cDNA of mnemiopsin, from Mnemiopsis leidyi. ► A unique EF-hand motif in which a conserved glycin is substituted with glutamate. ► The optimum pH 9.0, time course of regeneration is 15h. ► Kinetic behaviors are confirmed by theoretical and modeling investigation. Up to now, all reported Ca2+-regulated photoproteins, except for mnemiopsin, have been cloned and expressed in Escherichia coli. In this study, the cDNA for an isotype of mnemiopsin, from the ctenophore Mnemiopsis leidyi, has been cloned, sequenced, and functionally expressed. The full length cDNA encoding mnemiopsin of M. leidyi was 624bp open reading frame encoding a protein of 207 amino acid residues with calculated molecular mass of ∼24kDa. The deduced amino acid sequence showed 90% and 84% identity to berovine (from ctenophore Beroe abyssicola) and bolinopsin 2 (from the ctenophore Bolinopsis infundibulum) respectively. In contrast to all known EF-hand in photoproteins, a unique EF-hand motif was found in mnemiopsin, in which a conserved glycine is substituted with glutamic acid. According to the results, the optimum pH was 9.0, time course of regeneration was 15h and its Ca2+ sensitivity was lower than aequorin. Results of pKa calculation for ionizable residues, motif scan and hydrophobic interactions of cavity aromatic residues of mnemiopsin in comparison with aequorin showed different patterns in these two photoproteins. In addition, experimental results are confirmed with the theoretical studies.