Guanine nucleotide exchange factor (GEF) that activates RhoA, playing a role in the regulation of actin cytoskeleton organization (PubMed:36929019). Is involved in the regulation of vascular smooth muscle contractility (PubMed:12775584). Additionally, it negatively regulates excitatory synapse development by suppressing the synapse- promoting activity of EPHB2 (By similarity). Does not activate RAC1 or CDC42 (PubMed:36929019). {ECO:0000250|UniProtKB:Q5FWH6, ECO:0000269|PubMed:12775584, ECO:0000269|PubMed:36929019}. This is the function of ARHGEF15 (Rho guanine nucleotide exchange factor 15, ENSG00000198844).