Characterization of the interaction between beta(2)-glycoprotein I and calmodulin, and identification of a binding sequence in beta(2)-glycoprotein I

beta(2)-Glycoprotein I was shown to bind reversibly to calmodulin in a Ca2+-dependent manner with a 1:1 stoichiometry, a K-d of 3 x 10(-9) M and a Hill coefficient of 1.4. A sequence in beta(2)-glycoprotein I (Lys-Pro-Gly-Tyr-Val-Ser-Arg-Gly-Gly-Met-Arg-Lys-Phe-Ile-) limited by Cys-32 and Cys-47 is suggested to be the calmodulin-binding region. This sequence was the only one in beta(2)-glycoprotein I theoretically having the ability to form a basic amphiphilic alpha-helix typical of a calmodulin binding sequence. The peptide corresponding to this sequence was synthesized and found to inhibit the interaction between beta(2)-glycoprotein I and calmodulin with an IC50 value of 0.38 x [beta(2)-glycoprotein I] and to displace the beta(2)-glycoprotein I from the beta(2)-glycoprotein I/calmodulin complex with an IC50 value of 0.90 x [beta(2)-glycoprotein I].