Resolving the Interactions Between Class 3 Semaphorin Receptors in Live Cells

The plexin/neuropilin/semaphorin family of proteins is involved with tissue patterning in the developing embryo. These proteins play roles in cell migration and adhesion, but are also important in disease, including cancer angiogenesis and metastasis. While some structures of the soluble domains of these proteins have been determined, the conformations of full-length receptor complexes are just beginning to be studied, especially within the context of the cell plasma membrane. Pulsed-interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS) allows direct insight to the formation of protein-protein interactions in the membrane of live cells. Here we investigated the homodimerization of neuropilin-1, Plexin A2, Plexin A4, and Plexin D1. Consistent with previous studies, we found that neuropilin-1, Plexin A2 and Plexin A4 are dimers in the absence of exogenous ligand. Plexin D1, on the other hand, was monomeric under similar conditions, which had not been previously reported. We also found that Plexin A2 and A4 assemble into a heteromeric complex. Stimulation with Semaphorin 3A or Semaphorin 3C ligand neither disrupts nor enhances the dimerization of the receptors when they are expressed alone, suggesting that activation involves a conformational change rather than a shift in the monomer-dimer equilibrium. However, upon stimulation with Semaphorin 3C, Plexin D1 and neuropilin-1 form a heteromeric complex, while Semaphorin 3A does not induce a stable complex with these receptors. This analysis of interactions by PIE-FCCS provides a complementary approach to the existing structural and biochemical data that will aid in the development of new therapeutic strategies to target these receptors during disease.