Functions as a RAC1 guanine nucleotide exchange factor (GEF), activating Rac proteins by exchanging bound GDP for free GTP. Its activity is synergistically activated by phosphatidylinositol 3,4,5- trisphosphate and the beta gamma subunits of heterotrimeric G protein. Mediates the activation of RAC1 in a PI3K-dependent manner. May be an important mediator of Rac signaling, acting directly downstream of both G protein-coupled receptors and phosphoinositide 3-kinase. {ECO:0000269|PubMed:15304342, ECO:0000269|PubMed:15304343, ECO:0000269|PubMed:15897194}. This is the function of PREX2 (phosphatidylinositol-3,4,5-trisphosphate dependent Rac exchange factor 2, Ensembl gene identifier ENSG00000046889).