Identification of the Phospholipid-binding Site of Human β2-Glycoprotein I Domain V by Heteronuclear Magnetic Resonance

To understand the mechanism of the interaction between human β2-glycoprotein I (β2-GPI) and negatively charged phospholipids, we determined the three-dimensional solution structure of the fifth domain of β2-GPI by heteronuclear multidimensional NMR. The results showed that the molecule is composed of well-defined four anti-parallel β-strands and two short α-helices, as well as a long highly flexible loop. Backbone dynamic analysis demonstrated significant mobility of the flexible loop on a subnanosecond time scale. Structural modeling of the nicked fifth domain, in which the Lys317-Thr318 peptide bond was specifically cleaved, revealed the importance of this long C-terminal loop for the interaction between β2-GPI and negatively charged phospholipids. A titration experiment with the anionic surfactant SDS showed that this highly mobile loop, as well as the short β-hairpin between βC and βD strands, which is rich in positively charged residues, specifically interact with the surfactant. The mobile loop, together with the surrounding positively charged residues, probably construct the binding site for negatively charged phospholipids such as cardiolipin.