Identification of the phospholipid-binding site of human beta(2)-glycoprotein I domain V by heteronuclear magnetic resonance

To understand the mechanism of the interaction between human beta (2)-glyco protein I (beta (2)-GPI) and negatively charged phospholipids, we determine d the three-dimensional solution structure of the fifth domain of beta (2)- GPI by heteronuclear multidimensional NMR. The results showed that the mole cule is composed of well-defined four anti-parallel beta -strands and two s hort alpha -helices, as well as a long highly flexible loop. Backbone dynam ic analysis demonstrated significant mobility of the flexible loop on a sub nanosecond time scale. Structural modeling of the nicked fifth domain, in w hich the Lys317-Thr318 peptide bond was specifically cleaved, revealed the importance of this long C-terminal loop for the interaction between beta (2 )-GPI and negatively charged phospholipids. A titration experiment with the anionic surfactant SDS showed that this highly mobile loop, as well as the short beta -hairpin between betaC and betaD strands, which is rich in posi tively charged residues, specifically interact with the surfactant. The mob ile loop, together with the surrounding positively charged residues, probab ly construct the binding site for negatively charged phospholipids such as cardiolipin. (C) 2000 Academic Press.