Phosphatidylinositol 3,4,5-trisphosphate-dependent guanine nucleotide exchange factor (GEF) which plays a role in the activation of Rho GTPases RAC1, RhoA and CDC42 (PubMed:12651066, PubMed:15023524). Can regulate cell morphology in cooperation with activated RAC1 (By similarity). Involved in immune homeostasis by ensuring proper trafficking and availability of T-cell regulator CTLA-4 at T-cell surface (PubMed:31308374). Plays a role in Th2 (T helper cells) development and/or activation, perhaps by interfering with ZAP70 signaling (By similarity). {ECO:0000250|UniProtKB:Q8C2K1, ECO:0000269|PubMed:12651066, ECO:0000269|PubMed:15023524, ECO:0000269|PubMed:31308374}. This is the function of DEF6 (DEF6 guanine nucleotide exchange factor, ENSG00000023892).