Membrane bleb: A seesaw game of two small GTPases.

The plasma membrane is generally associated with underling actin cytoskeleton. When the plasma membrane detaches from actin filaments, it is expanded by the intracellular pressure and the spherical membrane protrusion which lacks underlying actin cortex, termed bleb, is formed. Bleb is widely used for migration across species; however, the molecular mechanism underlying membrane blebbing remains largely unknown. Our recent study revealed that 2 small GTPases, Rnd3 and RhoA, are important regulators of membrane blebbing. In the expanding blebs, Rnd3 is recruited to the plasma membrane and inhibits RhoA activity by activating RhoGAP. On the other hand, RhoA is activated at the retracting membrane and removes Rnd3 from plasma membrane by the activity of ROCK (Rho-associated protein kinase). ROCK is also important for the rapid reassembly of actin cortex and retraction of membrane blebs by activating Ezrin. We propose that a Rnd3 and RhoA cycle underlies the core machinery of continuous membrane blebbing.