Proteolytic cleavage of beta 2-glycoprotein I: reduction of antigenicity and the structural relationship

Binding of beta(2)-glycoprotein I (beta(2)-GPI)-dependent anticardiolipin a ntibodies (aCL) derived from antiphospholipid syndrome (APS) is significant ly reduced in aCL ELISA due to loss of the phospholipid (PL) binding proper ty of beta(2)-GPI by plasmin treatment. In the present study, the treatment generated a nicked form of alpha(2)-GPI and resulted in loss of antigenici ty for the autoantibodies detected in ELISA, using an PP-GPI directly adsor bed polyoxygenated carboxylated plate, the assay system of which was not re lated to PL binding. The nicked form bound to neither Cu2+-oxidized low-den sity lipoprotein (oxLDL) nor to beta(2)-GPI-specific lipid ligands isolated from oxLDL, the result being a complete loss of subsequent binding of anti -pg-GPI autoantibodies. The conformational change in the nicked domain V wa s predicted from its intact structure determined by an X-ray analysis (impl emented in Protein Data Bank: 1C1Z), molecular modeling and epitope mapping of a monoclonal anti-beta(2)-GPI antibody, i.e. Cof-18, which recognizes t he related structure. The analysis revealed that novel hydrophobic and elec trostatic interactions appeared in domain V after the cleavage, thereby aff ecting the PL binding of beta(2)-GPI, Such a conformational change may have important implications for exposure of cryptic epitopes located in the dom ains such as domain IV.