Plexins, semaphorins, and scatter factor receptors: A common root for cellguidance signals?

Semaphorins, the plexin family of semaphorin receptors, and scatter factor receptors share evolutionarily conserved protein modules, such as the semap horin domain and Met Related Sequences (MRS). All these proteins also have in common a role in mediating cell guidance cues. During development, scatt er factor receptors control cell migration, epithelial tubulogenesis, and n eurite extension. Semaphorins and their receptors are known signals for axo n guidance; they are also suspected to regulate developmental processes inv olving cell migration and morphogenesis, and have been implicated in immune function and tumor progression. Scatter factors and secreted semaphorins a re diffusible ligands, whereas membrane-bound semaphorins signal by cell-ce ll interaction. Cell guidance control by semaphorins requires plexins, alon e or in a receptor complex with neuropilins. Semaphorins, besides their rol e in axon guidance, are expected to have multiple functions in morphogenesi s and tissue remodeling by mediating cell-repelling cues through plexin rec eptors.