Mutant apoprotein of photoprotein with low calcium sensitivity

A mutant apoprotein comprising an amino acid sequence wherein the 23rd to 34th amino acids in the amino acid sequence of SEQ ID NO:

• 2 are substituted with an amino acid represented by formula I below:
• Xaa23-Xaa24-Xaa25-Xaa26-Xaa27-Xaa28-Xaa29-Xaa30-Xaa31-Xaa32-Xaa33-Xaa34, wherein:
• Xaa23 is Asp, Glu, Gln, Ser, Thr or Asn,
• Xaa24 is Lys, Arg, His, Leu or Thr,
• Xaa25 is Asp, Glu, Gln, Ser, Thr or Asn,
• Xaa26 is an optional amino acid,
• Xaa27 is Asp, Glu, Gln, Ser, Thr or Asn,
• Xaa28 is Gly,
• Xaa29 is an optional amino acid,
• Xaa30 is Ile, Leu or Val,
• Xaa31 is Asp, Glu, Gln, Ser, Thr or Asn,
• Xaa32 is an optional amino acid,
• Xaa33 is an optional amino acid, and,
• Xaa34 is Asp, Glu, Gln, Ser, Thr or Asn;
• having a function to bind to the peroxide of coelenterazine or the peroxide of a coelenterazine analogue to form a photoprotein capable of emitting light under the action of calcium ions;
• and,
• having a half decay time of the luminescence emitted by binding of the photoprotein to calcium ions being not less than 2 seconds.