Strukturforschung, D-82152 search and refined to an R-factor of 19.0%. The molecule consists of two Martinsried, Germany similar halves closely resembling annexin I connected by an a-helical segment and arranged perpendicular to each other. The calcium and membrane binding sites assigned by structural homology are therefore not located in the same plane. Analysis of the membrane-bound form of annexin VI by electron microscopy shows the two halves of the molecule coplanar with the membrane, but oriented differently to the crystal structure and suggesting a flexible arrangement. Ion channel activity has been found for annexin VI and the half molecules by electrophysiological experiments.