Insulin receptor tyrosine kinase substrate p53 (IRSp53) has been identified as 
an SH3 domain-containing adaptor that links Rac1 with a Wiskott-Aldrich syndrome 
family verprolin-homologous protein 2 (WAVE2) to induce lamellipodia or Cdc42 
with Mena to induce filopodia. The recruitment of these SH3-binding partners by 
IRSp53 is thought to be crucial for F-actin rearrangements. Here, we show that 
the N-terminal predicted helical stretch of 250 amino acids of IRSp53 is an 
evolutionarily conserved F-actin bundling domain involved in filopodium 
formation. Five proteins including IRSp53 and missing in metastasis (MIM) 
protein share this unique domain and are highly conserved in vertebrates. We 
named the conserved domain IRSp53/MIM homology domain (IMD). The IMD has domain 
relatives in invertebrates but does not show obvious homology to any known actin 
interacting proteins. The IMD alone, derived from either IRSp53 or MIM, induced 
filopodia in HeLa cells and the formation of tightly packed parallel F-actin 
bundles in vitro. These results suggest that IRSp53 and MIM belong to a novel 
actin bundling protein family. Furthermore, we found that filopodium-inducing 
IMD activity in the full-length IRSp53 was regulated by active Cdc42 and Rac1. 
The SH3 domain was not necessary for IMD-induced filopodium formation. Our 
results indicate that IRSp53, when activated by small GTPases, participates in 
F-actin reorganization not only in an SH3-dependent manner but also in a manner 
dependent on the activity of the IMD.