Identification of a novel interaction between the Ca(2+)-binding protein S100A11 and the Ca(2+)- and phospholipid-binding protein annexin A6

S100A11 is a member of the S100 family of EF-hand Ca²⁺-binding proteins, which is expressed in smooth muscle and other tissues. Ca²⁺ binding to S100A11 induces a conformational change that exposes a hydrophobic surface for interaction with target proteins. Affinity chromatography with immobilized S100A11 was used to isolate a 70-kDa protein from smooth muscle that bound to S100A11 in a Ca²⁺-dependent manner and was identified by mass spectrometry as annexin A6. Direct Ca²⁺-dependent interaction between S100A11 and annexin A6 was confirmed by affinity chromatography of the purified bacterially expressed proteins, by gel overlay of annexin A6 with purified S100A11, by chemical cross-linking, and by coprecipitation of S100A11 with annexin A6 bound to liposomes. The expression of S100A11 and annexin A6 in the same cell type was verified by RT-PCR and immunocytochemistry of isolated vascular smooth muscle cells. The site of binding of S100A11 on annexin A6 was investigated by partial tryptic digestion and deletion mutagenesis. The unique NH₂ terminal head region of annexin A6 was not required for S100A11 binding, but binding sites were identified in both NH₂- and COOH-terminal halves of the molecule. We hypothesize that an agonist-induced increase in cytosolic free Ca²⁺] leads to formation of a complex of S100A11 and annexin A6, which forms a physical connection between the plasma membrane and the cytoskeleton, or plays a role in the formation of signaling complexes at the level of the sarcolemma. smooth muscle; protein-protein interaction