Cdk5:p35 complex is associated with Plexin-A through the actived form of Fyn. CRMPs are the downstream substrates for Cdk5. Cdk5 phosphorylates serine 522 of CRMPs. Phosphorylation of CRMPs mediates the Sema3A induced growth cone collapse.<br>Collapsin response mediator proteins (CRMPs) are five homologous cytosolic phosphoproteins (CRMP1–5) involved in neuronal differentiation and axonal guidance. These members oligomerize and exist as tetramers.