Calcium-dependent binding of S100C to the N-terminal domain of annexin I

The annexin family of proteins is characterized by a conserved core domain that binds to Phospholipids in a CA(2+)-dependent manner. Each annexin also has a structurally distinct N-terminal domain that may impart functional specificity. To search for cellular proteins that interact with the N-terminal domain of annexin I, we constructed a fusion protein consisting of Glutathione S-transferase fused to Amino acids 2-47 of human annexin I (GST-AINT; AINT = annexin I N-terminal). Extracts from metabolically labeled A431 cells contained a single protein (M(r) approximately 10,000) that bound to GST-AINT in a CA(2+)-dependent manner. A synthetic peptide corresponding to Amino acids 2-18 of annexin I inhibited the binding of the 10-kDa protein to GST-AINT with half-maximal inhibition occurring at approximately 15 microM peptide. In cellular extracts, endogenous annexin I and the 10-kDa protein associated in a reversible CA(2+)-dependent manner. Experiments with Other annexins and with N-terminal truncated forms of annexin I indicated that the 10-kDa protein bound specifically to a site within the first 12 Amino acids of annexin I. The 10-kDa protein was purified from human placenta by hydrophobic and affinity chromatography. Amino acid sequence analysis indicated that the 10-kDa protein is the human homologue of S100C, a recently identified member of the S100 subfamily of EF-hand CA(2+)-binding proteins.