Structural and Functional Characterization of Endonexin II, a Calcium- and Phospholipid-Binding Protein

• Published in: Proceedings of the National Academy of Sciences - PNAS 1987-09, Vol.84 (17), p.6078-6082
• Main Authors: Schlaepfer, David D., Mehlman, Tevie, Burgess, Wilson H., Haigler, Harry T.
• Format: Article
• Language: English
• Subjects: Actins - metabolism; Amino Acid Sequence; Amino acids; Analytical, structural and metabolic biochemistry; Annexin A5; Annexins; Binding and carrier proteins; Binding sites; Biological and medical sciences; calcium; calcium-binding protein; Calcium-Binding Proteins - isolation & purification; Calcium-Binding Proteins - metabolism; Dialysis; endonexin II; Fundamental and applied biological sciences. Psychology; Humans; In Vitro Techniques; Liposomes; man; Molecular Weight; Phospholipids; Phospholipids - metabolism; placenta; Protein Binding; Proteins; Regional identity; Sequence homology; Ungulates
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.84.17.6078

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 >0.5 mM) and the affinity was increased by phosphatidylserine liposomes (Kd≃ 100 μ M). 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 >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.