Three-dimensional structure of membrane-bound annexin V. A correlative electron microscopy-X-ray crystallography study.

We have used electron microscopy to analyse the structure of wild-type human annexin V (recombinant and placental) and of several mutants (single and double point mutants) bound to monolayers composed of DOPS, DOPE, or brain extract (Folch fraction III). On these phospholipids and on DOPS/DOPC (3:1, w/w) protein trimers, as also found in 3-D crystals, assemble to form a hexagonal lattice with a unit vector length of about 18 nm. The resolution obtained in projection is 1.7 to 2.2 nm for wild-type and mutants. There are no significant differences between the annexin V mutants and the wild-type protein at this resolution. All proteins bind as trimers with their convex side harbouring the Ca(2+)-binding sites facing the membrane. A comparison of the 3-D reconstruction of annexin V wild-type with the high resolution crystal structure shows that the domain structure is preserved but the relative orientation of the modules (II/III) and (I/IV) is slightly changed so that the Ca(2+)-binding sites in all four domains (including the recently observed binding site in domain III) become coplanar to the membrane. The thickness of the molecule obtained in the 3-D reconstruction corresponds well with the thickness of the high resolution crystal structure indicative of peripheral binding of annexin V without substantial penetration of the membrane.