Guanine nucleotide exchange factor (GEF) for Rap1A, Rap2A and M-Ras GTPases. Does not interact with cAMP. Interacts with the second PDZ domain of human PTP1e. Upon binding to M-Ras, it translocates to the plasma membrane. Isoform 3 has highest expression levels in the brain, heart, liver, lung and placenta and is barely detectable in skeletal muscle, kidney and pancreas.