Multiple alignment of C2 domains from C2-PLDs. For comparison, three characterised C2 domains (from cytosolic phospholipase A2, phospholipase Cδ and Synaptotagmin I) are included (adopted from ). The domain consists of eight β-strands (here indicated by lines at the bottom of the alignment) linked by loop regions. Two basic topological variants of the C2 domain have been described, resulting from a circular permutation of the β-strands (details in ). The Topology I is exemplified here by the C2 domain from Synaptotagmin I, whereas cPLA2 and PLCδ1 exhibit the Topology II. C2 domains from PLDs are predicted to have the Topology II . The first number at each β-strand refers to the Topology II, the second number to the Topology I (the first β-strand of Synl in not shown here). Three loops containing Ca2+-coordinating ligands are indicated as Loop 1 through Loop 3. Black and grey background indicates more and less conserved positions, respectively. Residues, which bind Ca2+ by the side chains, are highlighted by a violet background. Other ligands for Ca2+ ions are provided by backbone carbonyls (the respective positions with a blue background). Only three Ca2+-binding positions (excluding backbone carbonyls) are shared by the characterised C2 domains, the first two occupied by aspartate residues, the third either by an aspartate or an asparagine residue. Other Ca2+ ligands are recruited from generally non-shared positions in distinct domains. Potential Ca2+-binding residues in the C2 domains from PLDs are shown on a red background (non-conserved residues contributing with the backbone carbonyls are not considered). No C2 domain from any PLD matches exactly any prototypic C2 domain with respect to the Ca2+-binding sites. See the text for details.