Coreceptor for SEMA3A and SEMA3F. Necessary for signaling by class 3 semaphorins and subsequent remodeling of the cytoskeleton. Plays a role in axon guidance in the developing nervous system. Regulates the migration of sympathetic neurons, but not of neural crest precursors. Required for normal dendrite spine morphology in pyramidal neurons. May play a role in regulating semaphorin-mediated programmed cell death in the developing nervous system. Class 3 semaphorins bind to a complex composed of a neuropilin and a plexin. The plexin modulates the affinity of the complex for specific semaphorins, and its cytoplasmic domain is required for the activation of down-stream signaling events in the cytoplasm. Detected in embryonic hindbrain, spinal cord, dorsal root ganglion, trigeminal ganglion and superior cervical ganglion. In newborns, detected throughout all layers of the hippocampus. No visible phenotype, but causes subtle changes in the central nervous system. Mice exhibit altered apical dendrite spine morphology in pyramidal neurons. Mice exhibit defasciculation of the facial branchiomotor nerve and of the ophthalmic branch of the trigeminus, with variable severity. The number of neurons in the dorsal root ganglion is higher than normal, probably due to reduced neuronal apoptosis. In mice lacking both Plxna3 and Plxna4, migrating neurons do not show the normal response to Sema3A and Sema3F and do not migrate away from these semaphorins (in vitro). Belongs to the plexin family.