Molecular Basis of Phospholipid Synergy in Promoting Blood Coagulation Reactions

Abstract 1109

Most steps in the blood coagulation cascade obligatorily take place on membrane surfaces and are dependent on the exposure of phosphatidylserine (PS). Many coagulation proteins bind to PS-containing membrane bilayers in a calcium-dependent manner via gamma-carboxyglutamate-rich (GLA) domains. Furthermore, certain phospholipids, most notably phosphatidylethanolamine (PE), strongly synergize with PS to promote clotting reactions. We have recently been investigating the molecular basis of lipid-Ca²⁺ and lipid-GLA interactions in blood clotting using solid-state NMR (SSNMR) studies that employ isotopically labeled PS. Our studies have revealed that Ca²⁺ induces two predominant conformations of the PS headgroup (Boettcher et al., Biochem. 50:2264–73, 2011). We now report the quantification of the intermolecular and intramolecular atomic distances within Ca²⁺-induced PS nanoclusters using SSNMR. We have also expanded SSNMR studies to phospholipid bilayers composed of trinary mixtures of PS, PC, and PE as a means to further investigate the impact of PE on PS headgroup conformations in an intact lipid bilayer (PE-PS synergy). Initial results suggest that incorporation of PE into the bilayer alters the Ca²⁺: PS ratio required to fully saturate the L-serine headgroups of PS. Additionally, PE appears to affect the predominant PS headgroup conformations in the presence of Ca²⁺, and may induce a third L-serine conformation. SSNMR studies utilizing isotopically labeled PE headgroups are currently underway which should provide novel information concerning PE's interactions with PS, Ca²⁺, and/or GLA domains.

Previous SSNMR studies from our lab have also indicated that a fraction of L-serine headgroups in PS adopt a new conformation when bovine prothrombin fragment 1 is bound to PS/PC bilayers (Tavoosi et al., J. Biol. Chem. 286:23247–53, 2011). These data support the idea that GLA domains contain an L-serine-specific binding site. We have now bound human prothrombin fragment 1 to membranes containing labeled PS headgroups for use in SSNMR studies. In addition, we have also employed surface plasmon resonance (SPR) experiments to investigate the interactions of both human and bovine prothrombin with bilayers containing PS and PE. These data demonstrate that binding affinities and binding footprints are similar for both prothrombin proteins, and that PE enhances binding affinities and increases the number of GLA binding sites for both proteins. Together, results from SSNMR and SPR studies suggest that different GLA domains use similar molecular mechanisms to interact with the membrane surface.