A NEUTRON SOLUTION SCATTERING STUDY OF THE STRUCTURE OF ANNEXIN-V AND ITS BINDING TO LIPID VESICLES

Low-angle neutron solution scattering has been used to study the structure of annexin-V and its interaction with small single-bilayer vesicles consisting of phosphatidylserine and phosphatidylcholine at a 33:66 (mol:mol) ratio. There was no evidence for a change in the state of aggregation of annexin-V, which remained as a monomer in the presence of 3 mm-free calcium. The only difference between presence and absence of free calcium was the increase of the radius of gyration, from 19(±)0·4) Å to 22(±)0·4) Å in 2H2O buffer and from 19·7(±)1·2) Å to 22·2(±1·2) Å in H2O buffer. The relative molecular weight, outer radius and average surface area per lipid of vesicles alone were respectively 2·5(±0·5) × 106, 127 Å and 90(±19) Å2. These parameters were not modified in the presence of free calcium, which testified to the absence of vesicle coalescence. The calcium-depedent binding of annexin-V was essentially interfacial and therefore did not alter significantly the structural characteristics of the vesicles. At saturation, 80(±10) annexin-V molecules were bound per vesicle, the available area per molecule eing 2500(±300) Å2 thus covering ∼28 lipid head groups. The protein shell was approximately 35Å thick. The apparent dissociation constant was probably less than 1 nm. These data contriubte to a more accurate definition of annexin-V as a possible probe of those cytodynamic events involving exposure of sequestered membrane aminophospholipids. © 1992.