The Rho family of small GTPases (RhoA, Rac1 and Cdc42) controls signal-transduction pathways that influence many aspects of cell behaviour, including cytoskeletal dynamics1–3. At the leading edge, Rac1 and Cdc42 promote cell motility through the formation of lamellipodia and filopodia, respectively. On the contrary, RhoA promotes the formation of contractile actin−myosin-containing stress fibres in the cell body and at the rear1, 2, 4. Here, we identify synaptopodin, an actinassociated protein, as a novel regulator of RhoA signalling and cell migration in kidney podocytes. We show that synaptopodin induces stress fibres by competitive blocking of Smurf1- mediated ubiquitination of RhoA, thereby preventing the targeting of RhoA for proteasomal degradation. Gene silencing of synaptopodin in kidney podocytes causes the loss of stress fibres and the formation of aberrant non-polarized filopodia and impairment of cell migration. Together, these data show that synaptopodin is essential for the integrity of the podocyte actin cytoskeleton and for the regulation of podocyte cell migration. © 2006, Nature Publishing Group Ltd. All rights reserved.