Human 67-kDa calelectrin contains a duplication of four repeats found in 35-kDa lipocortins

The 67-kDa calelectrin is the largest member of a family of Ca²⁺-binding proteins that associate with membranes and phospholipids in a Ca²⁺-dependent manner. Oligonucleotide probes based on peptide sequences obtained from purified bovine 67-kDa calelectrin were used to screen a human retina cDNA library, and the complete primary structure of human 67-kDa calelectrin was deduced by DNA sequence analysis. The protein consists of eight 68-amino acid repeats separated by linking sequences of variable lengths. It is highly similar to the human lipocortin I and II sequences, each of which contains four such repeats. The amino termini of the three proteins show no sequence similarity; however, in the repeated regions the proteins are 42-45% identical in sequence. Analysis of the 16 repeats from the three proteins provides insights into the structural basis for Ca²⁺-dependent phospholipid binding. These data place the calelectrins and the lipocortins into the same gene family and suggest that these proteins have similar functions and have evolved from a common ancestor.