Fig. 1. Molecular structure of annexin A1. Ribbon presentation showing the three-dimensional fold of the C backbone of annexin A1 in the presence (left) or absence (right) of Ca²⁺ ions (Rosengarth et al., 2001; Rosengarth and Luecke, 2003). The N-terminal domain (residues 1-40) is disordered in the X-ray structure of the Ca²⁺-bound annexin A1 and integrates into repeat 3 of the folded core domain in the Ca²⁺-free protein (depicted in yellow in the right-hand structure). Thus, upon Ca²⁺ binding, the N-terminal helix is expelled from the protein core and most likely becomes accessible for other interactions (Rosengarth and Luecke, 2003). In the Ca²⁺-bound conformation, the annexin can attach to membranes through its convex (upper) side, with the Ca²⁺ ions serving a bridging function. Figure kindly provided by Anja Rosengarth (University of California, Irvine).