title: Characterization of the interactions of the nephrin intracellular domain.
abstract: Nephrin is a signalling cell-cell adhesion protein of the Ig superfamily and the first identified component of the slit diaphragm that forms the critical and ultimate part of the glomerular ultrafiltration barrier. The extracellular domains of the nephrin molecules form a network of homophilic and heterophilic interactions building the structural scaffold of the slit diaphragm between the podocyte foot processes. The intracellular domain of nephrin is connected indirectly to the actin cytoskeleton, is tyrosine phosphorylated, and mediates signalling from the slit diaphragm into the podocytes. CD2AP, podocin, Fyn kinase, and phosphoinositide 3-kinase are reported intracellular interacting partners of nephrin, although the biological roles of these interactions are unclarified. To characterize the structural properties and protein-protein interactions of the nephrin intracellular domain, we produced a series of recombinant nephrin proteins. These were able to bind all previously identified ligands, although the interaction with CD2AP appeared to be of extremely low stoichiometry. Fyn phosphorylated nephrin proteins efficiently in vitro. This phosphorylation was required for the binding of phosphoinositide 3-kinase, and significantly enhanced binding of Fyn itself. A protein of 190 kDa was found to associate with the immobilized glutathione S-transferase-nephrin. Peptide mass fingerprinting and amino acid sequencing identified this protein as IQGAP1, an effector protein of small GTPases Rac1 and Cdc42 and a putative regulator of cell-cell adherens junctions. IQGAP1 is expressed in podocytes at significant levels, and could be found at the immediate vicinity of the slit diaphragm. However, further studies are needed to confirm the biological significance of this interaction and its occurrence in vivo.
authors.affiliation: Divisions of Matrix Biology, Department of Medical Biochemistry and Biophysics, Karolinska Institute, Stockholm, Sweden.
authors.forename: Xiao Li
authors.initials: XL
authors.lastname: Liu
authors.forename: Pekka
authors.initials: P
authors.lastname: Kilpeläinen
authors.forename: Ulf
authors.initials: U
authors.lastname: Hellman
authors.forename: Yi
authors.initials: Y
authors.lastname: Sun
authors.forename: Jorma
authors.initials: J
authors.lastname: Wartiovaara
authors.forename: Ekaterina
authors.initials: E
authors.lastname: Morgunova
authors.forename: Timo
authors.initials: T
authors.lastname: Pikkarainen
authors.forename: Kunimasa
authors.initials: K
authors.lastname: Yan
authors.forename: Anders P
authors.initials: AP
authors.lastname: Jonsson
authors.forename: Karl
authors.initials: K
authors.lastname: Tryggvason
mesh_terms: D000595:Amino Acid Sequence; D002460:Cell Line; D002846:Chromatography, Affinity; D006801:Humans; D007150:Immunohistochemistry; D008565:Membrane Proteins; D016253:Microscopy, Immunoelectron; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011485:Protein Binding; D011506:Proteins / Q000737:chemistry / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D020703:ras GTPase-Activating Proteins / Q000737:chemistry / Q000378:metabolism*
chemical_list: C101203:IQ motif containing GTPase activating protein 1; D008565:Membrane Proteins; D011506:Proteins; D011994:Recombinant Proteins; C113267:nephrin; D020703:ras GTPase-Activating Proteins