Molecular Basis for Lipid Specificity of the Coagulation Factor X Membrane-Binding Domain

Abstract

When a blood vessel is damaged, factor X (FX) and factor VIIa (FVIIa) bind to the cell membrane in a highly lipid-dependent manner and in complex with tissue factor initiate the blood coagulation cascade. Experimental information concerning the membrane-bound structure of FX at the atomic level and in particular its lipid specificity has remained elusive, largely due to the fluid nature of the cellular membrane. FX, along with FVIIa and several other proteins regulating the coagulation cascade, bind to the cell membrane using a γ-carboxyglutamate rich domain called the GLA domain. A set of 27 MD simulations with 9 independent initial configurations with respect to the membrane were performed to characterize an atomic model for the FX GLA domain bound to a lipid bilayer composed of a mixture of neutral or anionic lipids. The latter represent an important factor determining binding of coagulation proteins, e.g., in active platelets. Protein structures in each case began a distance of at least 5Å from membrane lipids. Application of the HMMM (highly mobile membrane mimetic) model in these simulations permitted spontaneous insertion and binding of the GLA domain to be captured. The final membrane-bound FX GLA model allows for detailed characterization of the orientation, insertion depth, and lipid interactions of the domain. Insight is provided into the molecular basis of lipid specificity of membrane binding in the FX GLA domain. All binding simulations converged to the same configuration despite differing initial orientations. Analysis of interactions between residues in the converged membrane-bound GLA domain and lipid functional groups allowed for potential phosphatidylserine specific binding sites to be identified. This new structural and dynamic information provides a major step towards understanding the role of lipid-protein interactions in regulating the blood coagulation cascade.