Abstract
The concave surface of the crescent-shaped Bin-amphiphysin-Rvs (BAR) domain is postulated to bind to the cell membrane to induce membrane deformation of a specific curvature. The Rac binding (RCB) domain/IRSp53-MIM homology domain (IMD) has a dimeric structure that is similar to the structure of the BAR domain; however, the RCB domain/IMD has a "zeppelin-shaped" dimer. Interestingly, the RCB domain/IMD of IRSp53 possesses Rac binding, membrane binding, and actin filament binding abilities. Here we report that the RCB domain/IMD of IRSp53 induces membrane deformation independent of the actin filaments in a Rac-dependent manner. In contrast to the BAR domain, the RCB domain/IMD did not cause long tubulation of the artificial liposomes; however, the Rac binding domain caused the formation of small buds on the liposomal surface. When expressed in cells, the Rac binding domain induced outward protrusion of the plasma membrane in a direction opposite to that induced by the BAR domain. Mapping of the amino acids responsible for membrane deformation suggests that the convex surface of the Rac binding domain binds to the membrane in a Rac-dependent manner, which may explain the mechanism of the membrane deformation induced by the RCB domain/IMD.
Highlights
The Rac binding (RCB) domain of IRSp53 shares its homology with the MIM protein, and this domain is named the IRSp53/MIM homology domain (IMD)
The helix bundle structure of the IRSp53 RCB domain was almost identical to the corresponding portion of the reported structure of the IRSp53 IMD, which is 22 residues longer than the RCB domain [8]
The RCB domain is an independent entity with a structure and surface electrostatic properties similar to that of the BAR domain (Fig. 1 and Fig. 2, A and B)
Summary
The RCB domain of IRSp53 shares its homology with the MIM (missing in metastasis) protein, and this domain is named the IRSp53/MIM homology domain (IMD) (residues 1–250 of IRSp53). IRSp53 RCB domain shows the actin filament binding activity, which was decreased by ϳ30% because of the BPM by Ala substitution (Fig. 2G).
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