Directional motility induced by epidermal growth factor requires Cdc42

Cell motility is actuated by a host of intracellular signaling cascades that result in movement of the cell in one direction, even without an external gradient. Phospholipase C-γ (PLCγ) has been shown to be important for growth factor-induced lamellipodial protrusion at the front of the cell while Cdc42 has been implicated in both filopodium formation at the leading edge and control of polarity of migrating cells. We asked whether these asymmetries in effector molecules may be linked. When we overexpressed either constitutively active, dominant negative, or GFP-tagged Cdc42, wild-type NR6 fibroblasts lost directionality, as expected. On epidermal growth factor (EGF) exposure these cells produced multiple, transient protrusions in every direction; these extensions failed to result in productive motility. GFP-tagged Cdc42 appeared transiently at edges of newly formed protrusions in EGF-stimulated cells while they moved haphazardly. While PLCγ is distributed throughout the cell, the ratio of active, tyrosyl-phosphorylated PLCγ was increased at the leading edge, where phosphatidylinositol (4,5)-bisphosphate (PIP 2) hydrolysis is concentrated. This co-localization of activities may be due to Cdc42 directing PLCγ to the cell front, as PLCγ associated with Cdc42 in an EGF-dependent manner. We conclude that Cdc42 controls cell polarity, likely in part, through its binding to active PLCγ.