FIGURE 1.

Semaphorins and their receptors. Semaphorins can be categorized into eight classes. Viral Sema is found in the genomes of certain DNA viruses; Sema1, Sema2, and Sema5c comprise the invertebrate Semaphorins; the other Semaphorin classes are found in vertebrates. Semaphorins are secreted (viral Sema, Sema2, and Sema3), membrane-spanning (Sema1, Sema4, Sema5, and Sema6) or glycosylphosphatidylinositol-anchored proteins (Sema7A). Semaphorins bind to Plxn receptors (PlxnA1–PlxnA4, PlxnB1–PlxnB3, PlxnC1, and PlxnD1) – see arrows for specific interactions. Sema3 require Npn for binding to PlxnA.