Drosophila Plexin B is a Sema-2a receptor required for axon guidance

Plexin receptors play a crucial role in the transduction of axonal guidance events elicited by semaphorin proteins. In Drosophila, Plexin A (PlexA) is a receptor for the transmembrane semaphorin semaphorin-1a (Sema-1a) and is required for motor and central nervous system (CNS) axon guidance in the developing embryonic nervous system. However, it remains unknown how PlexB functions during neural development and which ligands serve to activate this receptor. Here, we show that plexB, like plexA, is robustly expressed in the developing CNS and is required for motor and CNS axon pathfinding. PlexB and PlexA serve both distinct and shared neuronal guidance functions. We observe a physical association between these two plexin receptors in vivo and find that they can utilize common downstream signaling mechanisms. PlexB does not directly bind to the cytosolic semaphorin signaling component MICAL (molecule that interacts with CasL), but requires MICAL for certain axonal guidance functions. Ligand binding and genetic analyses demonstrate that PlexB is a receptor for the secreted semaphorin Sema-2a, suggesting that secreted and transmembrane semaphorins in Drosophila use PlexB and PlexA, respectively, for axon pathfinding during neural development. These results establish roles for PlexB in central and peripheral axon pathfinding, define a functional ligand for PlexB, and implicate common signaling events in plexin-mediated axonal guidance.