Sema4D binds Plexin-B1 to induce repulsive or attractive effects in neuronal and nonneuronal cells. Plexins constitute a large family of transmembrane proteins that function as receptors for semaphorins and their interaction governs cell adhesion and migration in a variety of tissues. All B-class plexins can interact with the receptor tyrosine kinases Met and ErbB2. The binding of Sema4D to plexin-B1 stimulates the intrinsic tyrosine kinase activity of Met, leading to the phosphorylation of both Plexin-B1 and Met. The phosphorylation of the plexin-B1/Met complex induced by Sema4D is crucial for epithelial cell migration and invasive growth. Sequence alignment reveals the presence of 13 conserved tyrosine residues (highly conserved sites 1918, 1953, 2038), but the specific tyrosine residues phosphorylated in the cytoplasmic domain of plexins in response to semaphorin stimulation have not yet been identified.