Guanine nucleotide exchange factor (GEF) that catalyzes the exchange of GDP for GTP. Promotes guanine nucleotide exchange on the Rho family members of small GTPases, like RHOA, RHOC, RAC1 and CDC42. Required for signal transduction pathways involved in the regulation of cytokinesis. Component of the centralspindlin complex that serves as a microtubule-dependent and Rho-mediated signaling required for the myosin contractile ring formation during the cell cycle cytokinesis. Regulates the translocation of RHOA from the central spindle to the equatorial region. Plays a role in the control of mitotic spindle assembly; regulates the activation of CDC42 in metaphase for the process of spindle fibers attachment to kinetochores before chromosome congression. Involved in the regulation of epithelial cell polarity; participates in the formation of epithelial tight junctions in a polarity complex PARD3-PARD6-protein kinase PRKCQ-dependent manner. Plays a role in the regulation of neurite outgrowth. Inhibits phenobarbital (PB)-induced NR1I3 nuclear translocation. Stimulates the activity of RAC1 through its association with the oncogenic PARD6A-PRKCI complex in cancer cells, thereby acting to coordinately drive tumor cell proliferation and invasion. Also stimulates genotoxic stress-induced RHOB activity in breast cancer cells leading to their cell death. Autoinhibited by the C-terminal PH domain which folds back and binds to the surface of the DH domain, blocking binding of RHOA to the catalytic center of the DH domain. The 2nd BRCT domain is also involved in inhibition, probably by helping to impede RHOA binding. Allosterically activated by binding of activated GTP-bound RHOA to the PH domain which stimulates the release of PH inhibition and promotes the binding of substrate RHOA to the catalytic center. Binding of phosphorylated RACGAP1 to the N-terminal BRCT domain-containing region also releases autoinhibition. Homodimer (By similarity). Homooligomer (By similarity). Found in the centralspindlin complex (By similarity). Interacts with NR1I3 (PubMed:17904126). Interacts (Thr-359 phosphorylated form) with PARD6A; the interaction is observed in cancer cells (By similarity). Interacts (Thr-359 phosphorylated form) with PRKCI; the interaction is observed in cancer cells (By similarity). Interacts with PKP4; the interaction is observed at the midbody (By similarity). Interacts with RACGAP1; the interaction is direct, occurs in a microtubule-dependent manner, occurs at anaphase and during cytokinesis, is inhibited in metaphase by phosphorylation of ECT2 on Thr-373 and is stimulated in early anaphase by dephosphorylation of ECT2 probably on Thr-373 through CDK1 activity (By similarity). Interacts with PLK1; the interaction is stimulated upon its phosphorylation on Thr-444 (By similarity). Interacts with RHOA; the interaction results in allosteric activation of ECT2 (By similarity). Interacts with KIF23, PARD3, PARD6B and PRKCQ (By similarity). Sequestered within the nucleus during interphase. Dispersed throughout the cytoplasm upon breakdown of the nuclear envelope during mitosis. Colocalizes with the centralspindlin complex to the mitotic spindles during anaphase/metaphase, the cleavage furrow during telophase and at the midbody at the end of cytokinesis. Colocalized with RhoA at the midbody. Its subcellular localization to tight junction is increased by calcium. Localized predominantly in the cytoplasm of numerous carcinoma cells (By similarity). Highest expression in testis. Also detectable in brain, kidney, liver and spleen. Expressed in the embryo at 16 dpc. Up-regulated by phenobarbital in the nucleus and cytoplasm of the liver. The BRCT domain 1 and 2 are required for the intramolecular interaction, but not for the intermolecular oligomerization. The BRCT domains negatively inhibit its GEF activity in interphase cells. The same BRCT domains may act as a positive regulatory motif for the completion of cytokinesis after the breakdown of nuclear membrane during mitosis (By similarity). Phosphorylated by PLK1 in vitro. Hyperphosphorylated during the G2 phase of the cell cycle. Phosphorylation at Thr-373 occurs during the G2/M phase, relieves its auto-inhibition status and stimulates its GEF activity. Phosphorylation at Thr-444 in G2/M phase is required for subsequent binding with PLK1 and Rho exchange activation. Dephosphorylated at the time of cytokinesis. Phosphorylation at Thr-359 is required for its transformation activity in cancer cells (By similarity). May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Erroneous CDS prediction.