Purification, biological properties and partial sequence analysis of 67-kDa calcimedin and its 34-kDa fragment from...

Calcimedin is a group of proteins, originally isolated from chicken gizzard, which are able to bind to several hydrophobic matrices in the presence of Ca2+. Although the molecular properties have been partially discovered, the physiological functions of calcimedins have not yet been clearly defined. In this study, we describe the isolation and characterization of 67-kDa calcimedin and its 34-kDa fragment from chicken gizzard. Both structural and functional studies establish that 67-kDa calcimedin is a member of the calpactin/lipocortin family: it displays phospholipase A2 inhibitory activity, Ca2(+)-dependent F-actin binding and phospholipid binding activity similar to those of calpactins (lipocortins). By comparing the sequence of 67-kDa calcimedin with the predicted sequence of 67-kDa calelectrin, we concluded that the primary structure of these 67-kDa proteins is highly conserved. In particular, the sequences GLGTDEGAIIXVLTQR and EGAGTDESTLIEIMATR conform with the annexin consensus sequence which is characteristic of the calpactin/lipocortin family. A 34-kDa fragment of 67-kDa calcimedin was also purified and their relatedness has been confirmed by antibody cross-reactivity. The sequence data further support that the 34-kDa fragment is derived from the C-terminal portion of 67-kDa calcimedin by limited proteolysis. The 34-kDa fragment, which contains the annexin consensus sequence, preserves the phospholipase A2 inhibitory activity, and binds F-actin and phospholipids.