SUMOylation of the GTPase Rac1 is required for optimal cell migration

The Rho-like GTPase Rac1 induces cytoskeletal rearrangements required for cell migration. Rac activity is regulated through a number of mechanisms, including control of nucleotide exchange and hydrolysis, regulation of subcellular localization, or modulation of protein expression levels1-3. Here, we identify the Small Ubiquitin-like MOdifier (SUMO) E3-ligase, PIAS3, as a new Rac1 interactor required for increased Rac activity and optimal cell migration in response to Hepatocyte Growth Factor (HGF) signalling. We show that Rac1 can be conjugated to SUMO-1 in response to HGF and that SUMOylation is enhanced by PIAS3. Moreover, we identify non-consensus sites within the polybasic region of Rac1 as the main locations for SUMO conjugation. We demonstrate that PIAS3-mediated SUMOylation of Rac1 controls its GTP-bound levels and its ability to stimulate lamellipodia, cell migration and invasion. This is the first time that a Ras superfamily member is found to be SUMOylated, providing a new insight into the regulation of these critical mediators of cell behaviour. Moreover, our data reveal a previously undescribed role for SUMO in the regulation of cell migration and invasion

Rac1-induced cell migration requires membrane recruitment of the nuclear oncogene SET

The Rho GTPase Rac1 controls cell adhesion and motility. The effector loop of Rac1 mediates interactions with downstream effectors, whereas its C-terminus binds the exchange factor β-Pix, which mediates Rac1 targeting and activation. Here, we report that Rac1, through its C-terminus, also binds the nuclear oncogene SET/I2PP2A, an inhibitor of the serine/threonine phosphatase PP2A. We found that SET translocates to the plasma membrane in cells that express active Rac1 as well as in migrating cells. Membrane targeting of SET stimulates cell migration in a Rac1-dependent manner. Conversely, reduction of SET expression inhibits Rac1-induced migration, indicating that efficient Rac1 signalling requires membrane recruitment of SET. The recruitment of the SET oncogene to the plasma membrane represents a new feature of Rac1 signalling. Our results suggest a model in which Rac1-stimulated cell motility requires both effector loop-based downstream signalling and recruitment of a signalling amplifier, that is, SET, through the hypervariable C-terminus