Hinge-bending motions in annexins: molecular dynamics and essential dynamics of apo-annexin V and of calcium bound annexin V and I.

Annexins are homologous proteins that bind to membranes in a calcium dependent manner, but for which precise physiological roles have yet to be defined. Most annexins are composed of a planar array of four homologous repeats, each containing five alpha-helices and associated into two modules. Annexin V forms a voltage-gated calcium channel in phospholipid bilayers. It has been proposed that the hydrophilic pore in the centre of the molecule may represent the ion conduction pathway and that a hinge movement in annexin V causes a variation of the inter-module angle and opens the calcium ion path. Here we present the results of molecular dynamics simulations of apo-annexin V and of calcium-bound annexin V and annexin I. The three simulations show significant differences in conformation and dynamics. The essential dynamics method was used to study the essential subspace of annexin V and showed that one of the essential motions corresponds to the postulated hinge motion. The hinge residues were located between repeats but belong to helices rather than to the links between helices. Calcium binding to annexin V led to a limitation of this hinge motion with more open conformations being favoured.