Structure of the human brain calcium-binding protein calretinin and its expression in bacteria

Calbindin D28k and calretinin are two closely related intracellular calcium-binding proteins belonging to the troponin C superfamily. Calbindin is known to be involved in the vitamin-D-dependent calcium absorption through intestinal and renal epithelia, while the function of neuronal calbindin and calretinin is poorly understood. Using antibodies directed against chick intestinal calbindin D28k, human calretinin cDNA clones were isolated from brain cDNA libraries. The sequence of the calretinin cDNA revealed an open reading frame of 271 codons coding for a protein of 31,520 Da, and sharing 58% identical residues with human calbindin D28k. Calretinin contains five presumably active and one presumably inactive calcium-binding domains. Comparison with the partial sequences available for chick and guinea pig calretinins revealed that the protein is highly conserved in evolution (evolutionary rate: 0.27 x 10(-9) amino acid-1 year-1). The calretinin message was detected in the brain, while absent from heart muscle, kidney, liver, lung, spleen, stomach and thyroid gland. Recombinant calretinin was expressed in Escherichia coli, and the calcium-binding properties were confirmed on both the natural and the recombinant proteins. Part of the human gene coding for calretinin was isolated and the region corresponding to the promoter and the first exon was sequenced.