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Schulze, JO, Quedenau, C, Roske, Y, Adam, T, Schüler, H, Behlke, J, Turnbull, AP, Sievert, V, Scheich, C, Mueller, U, Heinemann, U and Büssow, K (2008) Structural and functional characterization of human Iba proteins. FEBS J. 275:4627-40
Iba2 is a homolog of ionized calcium-binding adapter molecule 1 (Iba1), a 17-kDa protein that binds and cross-links filamentous actin (F-actin) and localizes to membrane ruffles and phagocytic cups. Here, we present the crystal structure of human Iba2 and its homodimerization properties, F-actin cross-linking activity, cellular localization and recruitment upon bacterial invasion in comparison with Iba1. The Iba2 structure comprises two central EF-hand motifs lacking bound Ca2+. Iba2 crystallized as a homodimer stabilized by a disulfide bridge and zinc ions. Analytical ultracentrifugation revealed a different mode of dimerization under reducing conditions that was independent of Ca2+. Furthermore, no binding of Ca2+ up to 0.1 mM was detected by equilibrium dialysis. Correspondingly, Iba EF-hand motifs lack residues essential for strong Ca2+ coordination. Sedimentation experiments and microscopy detected pronounced, indistinguishable F-actin binding and cross-linking activity of Iba1 and Iba2 with induction of F-actin bundles. Fluorescent Iba fusion proteins were expressed in HeLa cells and co-localized with F-actin. Iba1 was recruited into cellular projections to a larger extent than Iba2. Additionally, we studied Iba recruitment in a Shigella invasion model that induces cytoskeletal rearrangements. Both proteins were recruited into the bacterial invasion zone and Iba1 was again concentrated slightly higher in the cellular extensions.
Actins/metabolism; Amino Acid Sequence; Calcium/metabolism; Calcium-Binding Proteins/analysis; Calcium-Binding Proteins/chemistry; Calcium-Binding Proteins/metabolism; Crystallography, X-Ray; DNA-Binding Proteins/analysis; DNA-Binding Proteins/chemistry; DNA-Binding Proteins/metabolism; Dimerization; HeLa Cells; Humans; Microfilament Proteins/analysis; Microfilament Proteins/chemistry; Microfilament Proteins/metabolism; Models, Molecular; Molecular Sequence Data; Sequence Alignment; Shigella/pathogenicity