Involvement of Rac in actin cytoskeleton rearrangements induced by MIM-B.

Numerous scaffold proteins coordinate signals from the environment with actin-based protrusions during shape change and migration. Many scaffolds integrate signals from Rho-family GTPases to effect the assembly of specific actin structures. Here we investigate the mechanism of action MIM-B (missing in metastasis-B) on the actin cytoskeleton. MIM-B binds actin monomer through a WASP homology 2 motif, bundles actin filaments via an IRSp53/MIM domain, and is a long isoform of MIM, a proposed metastasis suppressor. We analysed the activity of MIM-B toward the actin cytoskeleton as well as its potential link to cancer metastasis. Endogenous MIM-B protein is widely expressed and its expression is maintained in various metastatic cell lines. MIM-B induces lamellipodia-like actin-rich protrusions. The IRSp53/MIM domain of MIM-B, as well as Rac activity are required to induce protrusions, but not the WASP homology 2 motif. MIM-B binds and activates Rac via its IRSp53/MIM domain, but this is not sufficient to induce lamellipodia. Finally, our data revealed that actin bundling and Rac-binding properties of MIM-B are not separable. Thus, MIM-B is unlikely to be a metastasis suppressor but acts as a scaffold protein that interacts with Rac, actin and actin-associated proteins to modulate lamellipodia formation.