The human ECT2 protooncogene encodes a guanine nucleotide exchange factor for the Rho GTPases and regulates cytokinesis. Although the oncogenic form of ECT2 contains an N-terminal truncation, it is not clear how the structural abnormality of ECT2 causes malignant transformation. Here we show that both the removal of the negative regulatory domain and alteration of subcellular localization are required to induce the oncogenic activity of ECT2. The transforming activity of oncogenic ECT2 was strongly inhibited by dominant negative Rho GTPases, suggesting the involvement of Rho GTPases in ECT2 transformation. Although deletion of the N-terminal cell cycle regulator-related domain (N) of ECT2 did not activate its transforming activity, removal of the small central domain (S), which contains two nuclear localization signals (NLSs), significantly induced the activity. The ECT2 N domain interacted with the catalytic domain and significantly inhibited the focus formation by oncogenic ECT2. Interestingly, the introduction of the NLS mutations in the S domain of N-terminally truncated ECT2 dramatically induced the transforming activity of this otherwise non-oncogenic derivative. Among the known Rho GTPases expressed in NIH 3T3 cells, RhoA was predominantly activated by oncogenic ECT2 in vivo. Therefore, the mislocalization of structurally altered ECT2 might cause the untimely activation of cytoplasmic Rho GTPases leading to the malignant transformation.
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