Ca2+ and membrane binding to annexin 3 modulate the structure and dynamics of its N terminus and domain III

Annexin 3 (ANX A3) represents similar to1% of the total protein of human neutrophils and promotes tight contact between membranes of isolated specific granules in vitro leading to their aggregation. Like for other annexins, the primary molecular events of the action of this protein is likely its binding to negatively charged phospholipid membranes in a Ca2+-dependent manner, via Ca2+-binding sites located on the convex side of the highly conserved core of the molecule. The conformation and dynamics of domain III can be affected by this process, as it was shown for other members of the family. The 20 amino-acid, N-terminal segment of the protein also could be affected and also might play a role in the modulation of its binding to the membranes. The structure and dynamics of these two regions were investigated by fluorescence of the two tryptophan residues of the protein (respectively, W190 in domain III and W5 in the N-terminal segment) in the wild type and in single-tryptophan mutants. By contrast to ANX A5, which shows a closed conformation and a buried W187 residue in the absence of Ca2+, domain III of ANX A3 exhibits an open conformation and a widely solvent-accessible W190 residue in the same conditions. This is in agreement with the three-dimensional structure of the ANX A3-E231A mutant lacking the bidentate Ca2+, ligand in domain III. Ca2+ in the millimolar concentration range provokes nevertheless a large mobility increase of the W190 residue, while interaction with the membranes reduces it slightly. In the N-terminal region, the W5 residue, inserted in the central pore of the protein, is weakly accessible to the solvent and less mobile than W190. Its amplitude of rotation increases upon binding of Ca2+ and returns to its original value when interacting with membranes. Ca2+ concentration for half binding of the W5A mutant to negatively charged membranes is similar to0.5 mM while it increases to similar to1 mM for the ANX A3 wild type and to similar to3 mM for the W190 ANX A3 mutant. In addition to the expected perturbation of the W 190 environment at the contact surface between the protein and the membrane bilayer, binding of the protein to Ca2+ and to membranes modulates the flexibility of the ANX A3 hinge region at the opposite of this interface and might affect its membrane permeabilizing properties.