Abstract:
:beta 2-Glycoprotein I (beta 2-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 beta 2-GPI that lacks aCL antibody cofactor activity. Analysis of the structural differences between the active and inactive forms enabled identification of the region of beta 2-GPI critically important for aCL cofactor activity. The active form of beta 2-GPI bound CL and displayed cofactor activity down to 1 microgram/ml. The inactive form failed to bind CL and possessed no cofactor activity even at concentrations up to 94 micrograms/ml, indicating that the ability of beta 2-GPI to bind lipids 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 beta 2-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 beta 2-GPI to bind lipid and for aCL cofactor activity. The clipped form of beta 2-GPI would not be suitable for use as aCL cofactor and its use may have led some investigators to conclude incorrectly that beta 2-GPI does not interact with aCL antibodies.
journal_name
Proc Natl Acad Sci U S Aauthors
Hunt JE,Simpson RJ,Krilis SAdoi
10.1073/pnas.90.6.2141subject
Has Abstractpub_date
1993-03-15 00:00:00pages
2141-5issue
6eissn
0027-8424issn
1091-6490journal_volume
90pub_type
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