The RAC Binding Domain/IRSp53-MIM Homology Domain of IRSp53 Induces RAC-dependent Membrane Deformation

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.

Filopodia: Complex models for simple rods

“…Overexpression of full length IRSp53 or of isolated IMDs induces filopodium-like membrane protrusions, the majority of which, however, differs significantly from canonical filopodia (Mattila et al, 2007, Millard et al, 2005, Suetsugu et al, 2006. First, ectopically overexpressed IRSp53/IMD localises to the entire plasma membrane enveloping these filopodialike structures (Figure 3), which is different from the localisation pattern of low expressors where IRSp53 is detected in tip complexes of both filopodia (Figure 3) and lamellipodia (compare Disanza et al, 2006, Millard et al, 2005, Nakagawa et al, 2003, Suetsugu et al, 2006.…”

“…First, ectopically overexpressed IRSp53/IMD localises to the entire plasma membrane enveloping these filopodialike structures (Figure 3), which is different from the localisation pattern of low expressors where IRSp53 is detected in tip complexes of both filopodia (Figure 3) and lamellipodia (compare Disanza et al, 2006, Millard et al, 2005, Nakagawa et al, 2003, Suetsugu et al, 2006. Notably, a convincing localisation of endogenous IRSp53 family porteins in protrusions of motile cells is still lacking, which would be instrumental to resolve the functional relevance of IMD-induced structures.…”

“…Notably, a convincing localisation of endogenous IRSp53 family porteins in protrusions of motile cells is still lacking, which would be instrumental to resolve the functional relevance of IMD-induced structures. Furthermore, Suetsugu et al (2006) have concluded that the F-actin content of these protrusions is lower than that of canonical filopodia, in which F-actin is densely packed in parallel bundles (outlined in Figure 3). Furthermore, canonical filopodia harbour at their tips an electron dense protein assembly that was shown to contain actin nucleating and elongating factors Rottner, 2006, Svitkina et al, 2003), which is not firmly established for IMD-induced structures.…”

“…using genetics, will be required. It will also be important to define how the IMD of IRSp53 family members contributes to filopodium formation, as multiple activities, like membrane binding, perhaps bending and tubulation or F-actin binding and bundling, have been attributed to this domain (Scita et al, 2008, Suetsugu et al, 2006. Moreover, IRSp53, but not the other family members, harbours a central CRIB (Cdc42/Rac interactive binding) motif rendering it a Cdc42 effector (Yamagishi et al, 2004).…”

Scaffold mediated regulation of MAPK signaling and cytoskeletal dynamics: A perspective

“…A family of related WAVE and WASP/Scar proteins bridge Rac and Cdc42 to the Arp2/3 complex that functions to nucleate actin polymerization and facilitate dendritic branching of actin filaments [27,28]. Rac through its binding to IRSp53 regulates WAVE dependent activation of Arp2/3 [29][30][31] to stimulate accumulation of densely branched actin filaments in lamellae and membrane ruffles that drive membrane protrusion [32]. Similarly, Cdc42 directly interacts with WASP to stimulate actin polymerization in filipodia [33,34].…”