Structural basis of the Ca2+-dependent association between S100C (S100A11) and its target, the N-terminal part of annexin I

Background: S100C (S100A11) is a member of the S100 calcium-binding protein family, the function of which is not yet entirely clear, but:may,include cytoskeleton assembly and dynamics. S100 proteins consist of two EF-hand calcium-binding motifs, connected by a flexible loop. Like several other members of the family, S100C forms a homodimer. A number of S100 proteins form complexes with annexins, another family of calcium-binding proteins that also bind to phospholipids, Structural studies have been undertaken to understand the basis of these interactions. Results: We have solved the crystal structure of a complex of calcium-loaded S100C with a synthetic peptide that corresponds to the first 14 residues-of the annexin I N terminus at 2.3 Angstrom resolution. We find a stoichiometry of one peptide per S100C monomer, the entire complex structure consisting of two peptides per S100C dimer, Each peptide, however, interacts with both monomers of the S100C dimer, The two S100C molecules of the dimer are linked,by a disulphide bridge. The structure is surprisingly close to that of the pll-annexin II N-terminal peptide complex solved previously. We have performed competition experiments to try to understand the specificity:of the S100-annexin interaction. Conclusions: By solving the structure of a second annexin N terminus-S100 protein complex, we confirmed a novel mode of interaction of S100 proteins with their target peptides; there is a one-to-one stoichiometry, where,the dimeric structure of the S100 protein is, nevertheless, essential for complex formation. Our structure can provide a model for a Ca2+-regulated annexin I-S100C heterotetramer, possibly involved in crosslinking membrane surfaces:or organising membranes during certain fusion events.