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Lack of Functional Similarity Between Complement Factor H and Anticardiolipin Cofactor, β2‐Glycoprotein I (Apolipoprotein H)
Author(s) -
PUURUNEN M.,
JOKIRANTA S.,
VAARALA O.,
MERI S.
Publication year - 1995
Publication title -
scandinavian journal of immunology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.934
H-Index - 88
eISSN - 1365-3083
pISSN - 0300-9475
DOI - 10.1111/j.1365-3083.1995.tb03694.x
Subject(s) - complement factor i , complement control protein , alternative complement pathway , activator (genetics) , glycoprotein , decay accelerating factor , complement system , factor h , cofactor , biochemistry , antibody , complement factor b , structural similarity , classical complement pathway , chemistry , biology , immunology , enzyme , receptor
Beta 2‐glycoprotein I (β2‐GPI) is a 50 kDa protein in human plasma composed of five repeating complement control protein modules thereby closely resembling complement factor H which has 20 such units. Both β2‐GPI and factor H (150 kDa) have binding sites for negatively charged polyions. β2‐GPI has been shown to act as a cofactor for antiphospholipid antibodies upon their binding to anionic phospholipids. In factor H the polyanion recognition site participates in the discrimination between alternative pathway activating and non‐activating surfaces. In light of the structural similarity between β2‐GPI and factor H we have examined whether β2‐GPI has a role in the alternative complement pathway recognition process. Both activators (zymosan) and non‐activators (sheep erythrocytes) of the alternative complement pathway were coated with C3b. Radiolabelled factor H was observed to recognize C3b on both surfaces, whereas β2‐GPI bound to neither. In competition experiments β2‐GPI could not prevent the association of 125 I‐H with either non‐activator or activator bound C3b. Conversely, factor H could not replace β2‐GPI as a cofactor for antiphospholipid antibodies upon their binding to anionic phospholipids. It is concluded that β2‐GPI and factor H, despite similarities in structure, exhibit distinct, non‐overlapping functions.