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 abili ty to bind to negatively charged phospholipid membranes in a Ca2+-dependent manner. Annexin A5, as well as other annexins, self-assembles into two-dim ensional (2D) ordered arrays upon binding to membranes, a property that has been proposed to have functional implications. Electron microscopy and ato mic force microscopy experiments have revealed that annexin A5 forms two ty pes of 2D crystals-with either p6 or p3 symmetry-that are both based on ann exin trimers, In this study, we describe three other crystal forms that coe xist with the p6 crystals. All crystal forms are made of the same building blocks, namely, dimers of trimers and trimers of trimers, A mechanistic mod el of the formation of the annexin AS 2D crystals is proposed. (C) 2001 Aca demic Press.