Complexes of plexin-A4 and plexin-D1 convey semaphorin-3C signals to induce cytoskeletal collapse in the absence of neuropilins. 
Class-3 semaphorin guidance factors bind to receptor complexes containing neuropilin and plexin receptors.  A semaphorin may bind to several receptor complexes containing somewhat different constituents, resulting in diverse effects on cell migration.  U87MG glioblastoma cells express both neuropilins and the four class-A plexins.  Here, we show that these cells respond to Sema3A or Sema3B by cytoskeletal collapse and cell contraction but fail to contract in response to Sema3C, Sema3D, Sema3G or Sema3E, even when class-A plexins are overexpressed in the cells.  In contrast, expression of recombinant plexin-D1 enabled contraction in response to these semaphorins.  Surprisingly, unlike Sema3D and Sema3G, Sema3C also induced the contraction and repulsion of plexin-D1-expressing U87MG cells in which both neuropilins were knocked out using CRISPR/Cas9.  In the absence of neuropilins, the EC50 of Sema3C was 5.5 times higher, indicating that the neuropilins function as enhancers of plexin-D1-mediated Sema3C signaling but are not absolutely required for Sema3C signal transduction.  Interestingly, in the absence of neuropilins, plexin-A4 formed complexes with plexin-D1, and was required in addition to plexin-D1 to enable Sema3C-induced signal transduction.