Ca ions facilitate the organization of the Annexin A2/S100A10 heterotetramer.

Annexin A2 (A2) is a member of the Annexin family, which contains Ca -regulated phospholipid-binding proteins. Annexins associate with S100 proteins to form heterotetramers. The A2/S100A10 heterotetramer (A2t) is the most extensively studied of these heterotetramers. It induces membrane microdomain formation, causes membrane budding, and facilitates proliferation of some cancers. In this work, the first molecular dynamics (MD) study on the complete A2t of 868 amino acids was performed. MD trajectories of more than 600 ns each were generated for complete A2t complexes with and without Ca ions. The outward extension of membrane-binding residues A2-K279 and A2-K281 was shown to be inhibited in the absence of Ca as they were captured by Ca -binding residue D322. F-actin binding residue A2-D339 was observed to occupy either an exposed or buried state in the absence of Ca , while it only occupied the buried state in the presence of Ca . The observed motions of the A2t subunits are highly organized with a strongly correlated central region which is negatively correlated with the periphery of the complex. The central region contains the S100A10 (p11) dimer, A2-N, and A2-I, while the periphery contains A2-II, A2-III, and A2-IV. Novel interactions between A2 and p11 were identified. A2 residues outside of A2-N (K80, R77, E82, and R145) had strong interactions with p11. Residue R145 of A2 may have a significant effect on the dynamics of the system, with its interaction resulting in asymmetric motions of A2. The presented results provide novel insights to inform future experimental studies.