The membrane-associated guanylate kinase protein MAGI-1 binds megalin and is present in glomerular podocytes

The transmembrane endocytic receptor glycoprotein 330/megalin thereafter re ferred to as megalin) is localized to the apical membrane domain of epithel ial cells, where it is involved in the uptake of proteins from extracellula r sources. The cytoplasmic domain of megalin contains amino acid motifs tha t have the potential to bind to other proteins, which may influence its loc alization or function. The yeast two-hybrid system was used to search for p roteins that bind to the cytoplasmic tail of megalin, and a protein fragmen t from a mouse embryonic cDNA library that contained a single PDZ domain wa s identified. This protein, which was named glycoprotein 330-associated pro tein (GASP), appears to be a truncated mouse counterpart of the human and r at proteins atrophin-1-interacting protein-1 and synaptic scaffolding molec ule, respectively. The interaction of GASP with megalin is mediated by the PDZ domain of GASP binding to the DSDV motif found at the carboxyl-terminus of megalin. A mutant version of megalin that lacks the terminal valine is unable to bind to GASP, illustrating the PDZ domain-dependent interaction b etween these two proteins. A close homolog of GASP, i.e., membrane-associat ed guanylate kinase with inverted orientation-1 (MAGI-1), is more ubiquitou s in its tissue distribution (including kidney) and is also able to specifi cally bind to megalin via its fifth PDZ domain. Immunofluorescence studies of adult kidney revealed that MAGI-1 is expressed in the glomerulus of the kidney, in a manner that parallels the expression of the podocyte-specific protein glomerular epithelial protein 1. Western analysis of endogenous MAG I-1 from glomerular preparations suggests that it is associated with the cy toskeleton and seems to be expressed in a different form, compared with cel l line-derived endogenous MAGI-1. The association of megalin with MAGI-1 ma y allow the assembly of a multiprotein complex, in which megalin may serve a nonendocytic function in glomerular podocytes.