Enables GTPase activator activity. Acts upstream of or within several processes, including labyrinthine layer blood vessel development; synaptic vesicle docking; and synaptonemal complex assembly. Located in cerebellar mossy fiber; cytosol; and synaptic membrane. Is expressed in labyrinthine zone; placenta; spongiotrophoblast; trophectoderm; and trophoblast giant cell. Orthologous to human SYDE1 (synapse defective Rho GTPase homolog 1). [provided by Alliance of Genome Resources, Apr 2022]