Spatial control of Cdc42 signalling by a GM130–RasGRF complex regulates polarity and tumorigenesis

The small GTPase Cdc42 is a key regulator of polarity, but little is known in mammals about its spatial regulation and the relevance of spatial Cdc42 pools for polarity. Here we report the identification of a GM130–RasGRF complex as a regulator of Cdc42 at the Golgi. Silencing GM130 results in RasGRF-dependent inhibition of the Golgi pool of Cdc42, but does not affect Cdc42 at the cell surface. Furthermore, active Cdc42 at the Golgi is important to sustain asymmetric front–rear Cdc42-GTP distribution in directionally migrating cells. Concurrent to Cdc42 inhibition, silencing GM130 also results in RasGRF-dependent Ras-ERK pathway activation. Moreover, depletion of GM130 is sufficient to induce E-cadherin downregulation, indicative of a loss in cell polarity and epithelial identity. Accordingly, GM130 expression is frequently lost in colorectal and breast cancer patients. These findings establish a previously unrecognized role for a GM130–RasGRF–Cdc42 connection in regulating polarity and tumorigenesis. The small GTPase Cdc42 is an important regulator of cell polarity that localizes to both the plasma membrane and to the Golgi. Baschieri et al. show that the Golgi pool of Cdc42 is regulated by GM130–RasGRF, and that deregulation of this pathway is associated with loss of polarity and tumorigenesis.