FRET studies with Factor X mutants provide insight into the topography of the membrane-bound Factor X/Xa

Abstract

FRET (fluorescence resonance energy transfer) studies have shown that the vitamin K-dependent coagulation proteases bind to membrane surfaces perpendicularly, positioning their active sites above the membrane surfaces. To investigate whether EGF (epidermal growth factor) domains of these proteases play a spacer function in this model of the membrane interaction, we used FRET to measure the distance between the donor fluorescein dye in the active sites of Fl–FPR (fluorescein–D-Phe-Pro-Arg-chloromethane)-inhibited fXa (activated Factor Xa) and its N-terminal EGF deletion mutant (fXa-desEGF1), and the acceptor OR (octadecylrhodamine) dye incorporated into phospholipid vesicles composed of 80% phosphatidylcholine and 20% phosphatidylserine. The average distance of closest approach (L) between fluorescein in the active site and OR at the vesicle surface was determined to be 56±1 Å (1 Å=0.1 nm) and 63±1 Å for fXa-desEGF1 compared with 72±2 Å and 75±1 Å for fXa, in the absence and presence of fVa (activated Factor V) respectively, assuming κ2=2/3. In comparison, an L value of 95±6 Å was obtained for a S195C mutant of fXa in the absence of fVa in which fluorescein was attached directly to Cys195 of fXa. These results suggest that (i) EGF1 plays a spacer function in holding the active site of fXa above the membrane surface, (ii) the average distance between fluorescein attached to Fl–FPR in the active site of fXa and OR at the vesicle surface may not reflect the actual distance of the active-site residue relative to the membrane surface, and (iii) fVa alters the orientation and/or the height of residue 195 above the membrane surface.