Structural and functional characterization of endonexin II, a calcium- and phospholipid-binding protein

A protein with an apparent Mr of 33,000 was previously purified from the EGTA eluate of a human placental particulate fraction. We now report the amino acid sequence of approximately one-third of this protein and show that it has extensive homology with a newly defined family of Ca2+-binding proteins termed annexins. The partial sequence of the placental protein could be aligned with the sequence of either lipocortin I or calpactin I such that 49% and 58%, respectively, of the residues were identical. A comparison of the partial sequences of the placental protein with the partial sequence of bovine endonexin revealed 74% sequence identity. Based on this close relationship, the placental protein was named endonexin II. Equilibrium dialysis showed that endonexin II bound Ca2+ (Kd greater than 0.5 mM) and the affinity was increased by phosphatidylserine liposomes (kd approximately equal to 100 microM). In addition, endonexin II bound to phosphatidylserine- and phosphatidylethanolamine-containing liposomes in a Ca2+-dependent manner, and the binding was cooperative with respect to Ca2+ concentration (Hill constant greater than 3). The Ca2+- and phospholipid-binding properties of endonexin II raise the possibility that each of the four internally repeated sequences that have been demonstrated within this family of proteins contains a Ca2+-binding site.