Structural and functional characterization of human Iba proteins


  • J.O. Schulze
  • C. Quedenau
  • Y. Roske
  • T. Adam
  • H. Schueler
  • J. Behlke
  • A.P. Turnbull
  • V. Sievert
  • C. Scheich
  • U. Mueller
  • U. Heinemann
  • K. Buessow


  • FEBS Journal


  • FEBS J 275 (18): 4627-4640


  • 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 Ca(2+). 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 Ca(2+). Furthermore, no binding of Ca(2+) up to 0.1 mm was detected by equilibrium dialysis. Correspondingly, Iba EF-hand motifs lack residues essential for strong Ca(2+) 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.