Fig. 7.

Models for how neuropilin-1 could be involved in Semaphorin signaling. A, Neuropilin-1 is unlikely to be acting as a type I receptor. Semaphorin signaling is more likely to involve a second component. Additional factor(s) may be present as preformed complexes with neuropilin-1 on the cell surface (B) or may be recruited into the neuropilin–semaphorin complex after ligand binding (C). Together, our data are most consistent with the third model.