Trimers, Dimers of Trimers, and Trimers of Trimers Are Common Building Blocks of Annexin A5 Two-dimensional Crystals

Annexin A5 is a member of a family of homologous proteins sharing the ability to bind to negatively charged phospholipid membranes in a Ca(2+)-dependent manner. Annexin A5, as well as other annexins, self-assembles into two-dimensional (2D) ordered arrays upon binding to membranes, a property that has been proposed to have functional implications. Electron microscopy and atomic force microscopy experiments have revealed that annexin A5 forms two types of 2D crystals-with either p6 or p3 symmetry-that are both based on annexin trimers. In this study, we describe three other crystal forms that coexist with the p6 crystals. All crystal forms are made of the same building blocks, namely, dimers of trimers and trimers of trimers. A mechanistic model of the formation of the annexin A5 2D crystals is proposed.