Structure and function of beta2-glycoprotein I

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beta2-Glycoprotein I (beta2GPI) is a cofactor in the recognition of the phospholipid antigen cardiolipin by anti-cardiolipin antibodies in autoimmune diseases such as systemic lupus erythematosus. beta2GPI (326 amino acids) consists of five repeating units (Domains I-V), each containing 60-80 amino acid residues, corresponding to the short consensus repeat of the complement control protein module. Recent studies suggest that Domain V plays important roles in the binding to cardiolipin and expression of cofactor activity. We studied the structure-function relationship of beta2GPI by preparing various derivatives.1.We examined the interactions of various forms of bovine beta2GPI with phospholipid liposomes under different conditions. The results indicate that the N-terminal as well as C-terminal domains have an important role in the interaction of beta2GPI with cardiolipin, and that the three residual domains containing sialic acid have no significant effect on the interaction.2.We constructed a high-level expression system for human Domain V using a methylotrophic yeast, Pichia pastoris. We found that the recombinant protein as the native disulfide bonds and a proper folded structure. For the three dimensional structure determination by NMR,with this expression system, we prepared 15N and 13C labeled Domain V.The NMR studies are on going.3.Conformation, stability, and liposome-binding activity of the recombinant Domain V were characterized and compared with those of various beta2GPI derivatives. The results suggested that the region including Trp76-Thr78 has a critical role in binding to cardiolipin.4.A nicked form of domain V in beta2GPI has been shown to have a lower ability to cardiolipin. Using the recombinant Domain V,we found that plasmin can produce the nicked form of domain V,suggesting the biological significance.