Specific GEF for RhoA activation. Does not activate RAC1 or CDC42. Regulates vascular smooth muscle contractility. Negatively regulates excitatory synapse development by suppressing the synapse-promoting activity of EPHB2. Interacts with EPHA4 (By similarity). Interacts with EPHB2. Expressed exclusively in dendrites of the developing hippocampus. At P12, expressed is detected in the CA1 region and the dentate gyrus of the hippocampus. Highest levels in hippocampus are found at postnatal day 3 prior to maximal synapse formation and decrease as synapse formation peaks in the postnatal period (at protein level). Phosphorylated on tyrosine residues upon EFNA1 stimulation. EPHB2-dependent phosphorylation at Tyr-361 triggers UBE3A-mediated ubiquitination. Ubiquitinated; UBE3A-mediated ubiquitination and degradation by the proteasome promotes EFNB1-dependent synapse formation. Significant decrease in RHOA activation in brain extracts. In a mouse model of Angelman syndrome where Ube3a levels are reduced, levels of Arhgef15 are significantly increased and ubiquitination is reduced compared to wild-type litter mates.