Guanine nucleotide-releasing protein that binds to SH3 domain of CRK and GRB2/ASH. Transduces signals from CRK to activate RAS. Involved in cell branching and adhesion mediated by BCAR1-CRK-RAPGEF1 signaling and activation of RAP1 (PubMed:12432078). Plays a role in the establishment of basal endothelial barrier function. Plays a role in nerve growth factor (NGF)-induced sustained activation of Rap1 and neurite outgrowth. Interacts with HCK (via SH3-binding sites) (PubMed:14551197). Interacts with CRK (via SH3-binding sites) (PubMed:8662907). Ubiquitously expressed in adult and fetus. Expression is high in adult skeletal muscle and placenta and in fetal brain and heart. Low levels of expression in adult and fetal liver. Phosphorylation at Tyr-504 enhances activity as Rap guanine nucleotide exchange factor.