Membrane-associated guanylate kinase-interacting protein (MAGUIN)-1 was identified as a protein interacting with synaptic scaffolding molecule (S-SCAM) and postsynaptic density (PSD)-95/synapse-associated protein (SAP)SO. MAGUIN-1 has a chimerical molecular structure composed of one sterile alpha moth, one PSD-95/Dlg-A/ZO-1 (PDZ), and one pleckstrin homology (PH) domain, and interacts with the PDZ domains of S SCAM and PSD-95/SAP90 via its carboxyl-terminal PDZ-binding motif. MAGUIN-1 is considered as a mammalian homologue of Drosophila CNK which is a Raf-interacting protein implicated in the regulation of eye development. Here we have tested whether MAGUIN-1 interacts directly with Raf-l. MAGUIN-1 and Raf-l were coimmunoprecipitated hem rat brain. MAGUIN-1 binds to the kinase domain of Raf-l, and Raf-l binds to the mid die region of MAGUIN-1 containing the PH domain. However, in contrast to the dominant active mutant of Ki-Ras, which interacts with Raf-l, recruits it to the plasma membrane from the cytosol, and activates it, MAGUIN-1 neither activates Raf-l nor recruits it to the plasma membrane. MAGUIN-1 may link Raf-l to components of synapses assembled by PSD-95/SAP90 and S-SCAM.