Crystal structure of bovine annexin VI in a calcium-bound state 1 Agustin J. Avila-Sakar a;b; *, Carl E. Creutz a , Robert H. Kretsinger b a Department of Pharmacology, University of Virginia, Charlottesville, VA 22908, USA b Department of Biology, University of Virginia, Charlottesville, VA 22908, USA Received 2 February 1998; accepted 5 May 1998 Abstract The crystal structure of a calcium-bound form of bovine annexin VI has been determined with X-ray diffraction data to 2.9 A î by molecular replacement. Six Ca 2 ions were found, five in AB loops, one in a DE loop. Two loops (II-AB, which binds calcium, and V-AB, which does not) have conformations that differ significantly from those in calcium-free, human recombinant annexin VI. There are only small differences between the calci- and the apo-annexin VI in the rest of the molecule. Calcium by itself does not promote a major conformational change. ß 1998 Elsevier Science B.V. All rights reserved. Keywords : Annexin ; Calcium ; X-ray crystallography 1. Introduction Annexin VI (68 kDa) is a member of the annexin family of calcium-dependent membrane-binding pro- teins [1]. It consists of two disk-shaped lobes and a linker [2,3]. Each lobe has four V70 amino acid repeats characteristic of all members of the family, which fold into the compact structure commonly re- ferred to as the annexin `core' [4]. The lobes are roughly perpendicular to one another. There is an extensive area of contact between them, which also involves the connecting linker [2]. The annexin `core' has been described in detail in several literature reports and reviews (see, for exam- ple, [1,4]). Each of the four copies of the V70 amino acid repeat constitutes a domain with ¢ve K-helices (A^E). Potential calcium-binding sites occur in the loop between helices A and B, and in the loop be- tween helices D and E. These calcium-binding motifs are di¡erent from the EF-hand [5]. The AB calcium sites in annexins are similar to the calcium sites of phospholipase A 2 [6]. Typically, three Ca 2 ligands are carbonyl oxygens from the AB loop, whose con- sensus sequence has alternate glycines that enable a highly curved conformation. A bidentate carboxylate contributes another two ligands. This residue is lo- cated about 40 residues away from the AB loop in the linear sequence and is generally called the `cap- ping residue' or `cap' [6]. In crystal structures, the remaining coordinating oxygens are generally con- tributed by water molecules or other components of the solvent. The DE calcium sites also typically have three carbonyl ligands and a carboxylate, but this last one is close to the carbonyls in the linear sequence. All calcium sites found so far in annexins are on one, slightly convex, face of the core. This is the face through which annexins presumably bind to 0167-4838 / 98 / $ ^ see front matter ß 1998 Elsevier Science B.V. All rights reserved. PII:S0167-4838(98)00111-3 * Corresponding author. Fax: +1 (804) 982 3878; E-mail : aa4n@virginia.edu 1 The coordinates have been deposited in the Brookhaven Pro- tein Data Bank under code 1AVC. Biochimica et Biophysica Acta 1387 (1998) 103^116