X-ray Structures of the Microglia/Macrophage-specific Protein Iba1 from Human and Mouse Demonstrate Novel Molecular Conformation Change Induced by Calcium binding

The ionized calcium-binding adaptor molecule 1 (Iba1) with 147 amino acid residues has been identified as a calcium-binding protein, expressed specifically in microglia/macrophages, and is expected to be a key factor in membrane ruffling, which is a typical feature of activated microglia. We have determined the crystal structure of human Iba1 in a Ca2+-free form and mouse Iba1 in a Ca2+-bound form, to a resolution of 1.9 Å and 2.1 Å, respectively. X-ray structures of Iba1 revealed a compact, single-domain protein with two EF-hand motifs, showing similarity in overall topology to partial structures of the classical EF-hand proteins troponin C and calmodulin. In mouse Iba1, the second EF-hand contains a bound Ca2+, but the first EF-hand does not, which is often the case in S100 proteins, suggesting that Iba1 has S100 protein-like EF-hands. The molecular conformational change induced by Ca2+-binding of Iba1 is different from that found in the classical EF-hand proteins and/or S100 proteins, which demonstrates that Iba1 has an unique molecular switching mechanism dependent on Ca2+-binding, to interact with target molecules.