Oncogenic Activity of Ect2 Is Regulated through Protein Kinase Cι-mediated Phosphorylation

The Rho GTPase guanine nucleotide exchange factor Ect2 is genetically and biochemically linked to the PKC iota oncogene in non-small cell lung cancer (NSCLC). Ect2 is overexpressed and mislocalized to the cytoplasm of NSCLC cells where it binds the oncogenic PKC iota-Par6 complex, leading to activation of the Rac1 small GTPase. Here, we identify a previously uncharacterized phosphorylation site on Ect2, threonine 328, that serves to regulate the oncogenic activity of Ect2 in NSCLC cells. PKC iota directly phosphorylates Ect2 at Thr-328 in vitro, and RNAi-mediated knockdown of either PKC iota or Par6 leads to a decrease in phospho-Thr-328 Ect2, indicating that PKC iota regulates Thr-328 Ect2 phosphorylation in NSCLC cells. Both wild-type Ect2 and a phosphomimetic T328D Ect2 mutant bind the PKC iota-Par6 complex, activate Rac1, and restore transformed growth and invasion when expressed in NSCLC cells made deficient in endogenous Ect2 by RNAi-mediated knockdown. In contrast, a phosphorylation-deficient T328A Ect2 mutant fails to bind the PKC iota-Par6 complex, activate Rac1, or restore transformation. Our data support a model in which PKC iota-mediated phosphorylation regulates Ect2 binding to the oncogenic PKC iota-Par6 complex thereby activating Rac1 activity and driving transformed growth and invasion.