Determination of molecular groups involved in the interaction of annexin A5 with lipid membrane models at the air-water interface

Annexin A5 (AnxA5) is a member of a family of homologous proteins sharing the ability to bind to negatively charged phospholipid membranes in a Ca²⁺-dependent manner. In this paper, we used polarization-modulated infrared reflection absorption spectroscopy (PMIRRAS), Brewster angle microscopy (BAM), and ellipsometry to investigate changes both in the structure of AnxA5 and phospholipid head groups associated with membrane binding. We found that the secondary structure of AnxA5 in the AnxA5/Ca²⁺/lipid ternary complex is conserved, mainly in α-helices and the average orientation of the α-helices of the protein is slightly tilted with respect to the normal to the phospholipid monolayer. Upon interaction between AnxA5 and phospholipids, a shift of the ν_as PO₂⁻ band is observed by PMIRRAS. This reveals that the phosphate group is the main group involved in the binding of AnxA5 to phospholipids via Ca²⁺ ions, even when some carboxylate groups are accessible (PS). PMIRRAS spectra also indicate a change of carboxylate orientation in the aspartate and glutamate residues implicated in the association of the AnxA5, which could be linked to the 2D crystallization of protein under the phospholipid monolayer. Finally, we demonstrated that the interaction of AnxA5 with pure carboxylate groups of an oleic acid monolayer is possible, but the orientation of the protein under the lipid is completely different.