X-ray structures of the microglia/macrophage-specific protein Iba1 from human and mouse demonstrate novel molecular
Mitsugu Yamada1, Keiko Ohsawa, Yoshinori Imai
1Information Technology Center and Faculty of Medicine, Kagawa University, 1750-1 Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0793, Japan.
Abstract:
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 Ca(2+)-free form and mouse Iba1 in a Ca(2+)-bound form, to a resolution of 1.9 A and 2.1 A, 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 Ca(2+), 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 Ca(2+)-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 Ca(2+)-binding, to interact with target molecules.
Insights
Ionized calcium-binding adaptor molecule 1 (Iba1) is a microglial protein crucial for cell activation. Its unique calcium-binding mechanism, revealed by crystal structures, differs from classical EF-hand proteins, suggesting a novel molecular switching function.
Area of Science:
- Biochemistry
- Structural Biology
- Neuroscience
Background:
- Ionized calcium-binding adaptor molecule 1 (Iba1) is a calcium-binding protein specifically expressed in microglia/macrophages.
- Iba1 is implicated in membrane ruffling, a key characteristic of activated microglia.
Purpose of the Study:
- To determine the crystal structure of human and mouse Iba1.
- To elucidate the calcium-binding mechanism and conformational changes of Iba1.
Main Methods:
- X-ray crystallography was used to determine the structures of human Iba1 (Ca(2+)-free) and mouse Iba1 (Ca(2+)-bound).
- Structural analysis focused on EF-hand motifs and calcium-binding interactions.
Main Results:
- Crystal structures revealed Iba1 as a compact, single-domain protein with two EF-hand motifs, similar to troponin C and calmodulin.
- Mouse Iba1 showed Ca(2+) bound to the second EF-hand but not the first, characteristic of S100 proteins.
- Calcium binding induced a unique conformational change distinct from classical EF-hand and S100 proteins.
Conclusions:
- Iba1 possesses S100 protein-like EF-hands.
- Iba1 exhibits a unique calcium-dependent molecular switching mechanism for target molecule interaction.
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