High-resolution structures of annexin A5 in a two-dimensional array

[Display omitted] •The crystal structures of human annexin A5 in its 2D array conformation were determined.•Structural analyses suggest conformational changes in annexin A5 into sheet-like arrays in response to Ca2+ concentration.•Provide insight into the roles of Ca2+ and phosphatidyl serine binding during the formation of 2D arrays. Annexins are soluble cytosolic proteins that bind to cell membranes. Annexin A5 self-assembles into a two-dimensional (2D) array and prevents cell rupture by attaching to damaged membranes. However, this process is not fully understood at the molecular level. In this study, we determined the crystal structures of annexin A5 with and without calcium (Ca2+) and confirmed the Ca2+-dependent outward motion of a tryptophan residue. Strikingly, the two structures exhibited the same crystal packing and 2D arrangement into a p3 lattice, which agrees well with the results of low-resolution structural imaging. High-resolution structures indicated that a three-fold interaction near the tryptophan residue is important for mediating the formation of the p3 lattice. A hypothesis on the promotion of p3 lattice formation by phosphatidyl serine (PS) is also suggested. This study provides molecular insight into how annexins modulate the physical properties of cell membranes as a function of Ca2+ concentration and the phospholipid composition of the membrane.