Cell junction-associated proteins IQGAP1, MAGI-2, CASK, spectrins, and α-actinin are components of the nephrin multiprotein complex

Nephrin is a cell surface receptor of the Ig superfamily that localizes to slit diaphragms, the specialized junctions between the interdigitating foot processes of the glomerular epithelium (podocytes) in the kidney. Mutations in the NPHS1 gene encoding nephrin lead to proteinuria and congenital nephrotic syndrome, indicating that nephrin is essential for normal glomerular development and function. To identify nephrin-binding proteins, we performed mass spectrometry on proteins obtained from pull-down assays with GST-nephrin cytoplasmic domain. Nephrin specifically pulled down six proteins from glomerular lysates, MAGI-2/S-SCAM (membrane-associated guanylate kinase inverted 2/synaptic scaffolding molecule), IQGAP1 (IQ motif-containing GTPase-activating protein 1), CASK (calcium/calmodulin-dependent serine protein kinase), a-actinin, all spectrin, and beta II spectrin. All of these scaffolding proteins are often associated with cell junctions. By immunofluorescence these proteins are expressed in glomerular epithelial cells, where they colocalize with nephrin in the foot processes. During glomerular development, IQGAP1 is expressed in the junctional complexes between the earliest identifiable podocytes, MAGI-2/S-SCAM is first detected in junctional complexes in podocytes after their migration to the base of the cells. Thus, the nephrin-slit diaphragm protein complex contains a group of scaffolding proteins that function to connect junctional membrane proteins to the actin cytoskeleton and signaling cascades. Despite their special morphology and function, there is considerable compositional similarity between the podocyte slit diaphragm and typical junctional complexes of other epithelial cells.