Expression and characterization of EF-hand I loop mutants of aequorin replaced with other loop sequences of Ca2+-binding proteins: an approach to studying the EF-hand motif of proteins

摘要

The binding properties of Ca2+ to EF-hand I of aequorin (AQ) were characterized by replacing the loop sequence of EF-hand I (AQ[I]) with other known loop sequences of Ca2+-binding proteins, including photoproteins (aequorin, clytin-I, clytin-II and mitrocomin), Renilla luciferin-binding protein (RLBP) and calmodulin (CaM). For evaluation of the binding affinity of Ca2+ to AQ[I] mutants, the half-decay time of the maximum intensity in the luminescence reaction triggered by Ca2+ was used as an indicator and 22 kinds of AQ[I] mutants were expressed in Escherichia coli cells. AQ[I] mutants replaced with the EF-hand I and EF-hand III from photoproteins showed sufficient luminescence activity, but it was not shown by other EF-hands from RLBP and CaM. An AQ[I] mutant with a lysine or arginine residue at the second position of the non-conserved amino acid residue showed a slow-decay pattern of luminescence, indicating that the Ca2+-binding affinity to aequorin was reduced by a positive charge at the second position of the loop sequence. The specific loop sequence of the EF-hand I motif in aequorin caused the specific Ca2+-triggered luminescence pattern.