Identification of a Region of β2-Glycoprotein I Critical for Lipid Binding and Anti-Cardiolipin Antibody Cofactor Activity

β2-Glycoprotein I (β2-GPI), a phospholipid-binding plasma protein, is an absolute requirement (cofactor) for the binding of autoimmune-type anti-cardiolipin (aCL) antibodies to cardiolipin (CL). The nature of this cofactor activity and the specific regions of the molecule involved have not yet been determined. We have identified a preparation of β2-GPI that lacks aCL antibody cofactor activity. Analysis of the structural differences between the active and inactive forms enabled identification of the region of β2-GPI critically important for aCL cofactor activity. The active form of β2-GPI bound CL and displayed cofactor activity down to 1 μ g/ml. The inactive form failed to bind CL and possessed no cofactor activity even at concentrations up to 94 μ g/ml, indicating that the ability of β2-GPI to bind lipid is an absolute requirement for aCL cofactor activity. Both forms possessed identical N-terminal sequences and were recognized as essentially immunoreactively identical by polyclonal antisera to β2-GPI. However, the inactive form has undergone proteolytic cleavage and exists primarily as a "clipped" molecule, the polypeptide chain being cleaved between Lys-317 and Thr-318 (a potential thrombin cleavage site), with the two cleaved segments linked as a disulfide-bonded complex. This indicates that the C-terminal region is critically important for β2-GPI to bind lipid and for aCL cofactor activity. The clipped form of β2-GPI would not be suitable for use as aCL cofactor and its use may have led some investigators to conclude incorrectly that β2-GPI does not interact with aCL antibodies.