Identification of MAGI-3 as a transforming growth factor-α tail binding protein

The cytoplasmic domain of the transforming growth factor-a precursor (proTGFalpha) contains a C-terminal PSD-95/SAP90. Discs Large. and Zona Occludens-1 (PDZ) recognition motif (TVV). By yeast two-hybrid screening of a mouse embryo library. we have found that a third member of a family of PDZ-containing proteins.. membrane associated guanylate kinase inverted-3 (MAGI-3). binds to TGFalpha's TVV MAGI-3 is widely expressed in multiple mouse tissues, including brain. Immunolocalization showed that MAGI-3 and TGFalpha were colocalized in neurons in the cortex and dentate gyrus, as well as in ependyimal cells and some astrocytes. In vitro, proTGFalpha bound the PDZ-1 domain of MAGI-3 and MAGI-2, but not MAGI-1. MAGI-3 and the 17-kDa cell surface form of proTGFalpha interact transiently in MDCK cells stably transfected with both MAGI-3 and human proTGFalpha cDNAs. MAGI-3 and wild-type proTGFalpha colocalize at the cell surface. In contrast MAGI-3 forms a stable complex with membrane-fixed TGFalpha early in the secretory pathway and interacts with immature and cell surface forms of membrane-fixed TGFalpha, Overexpression of MAGI-3 resulted in increased levels of TGFalpha: in the basolateral medium of polarized MDCK cells, suggesting that MAGI-3 has a role in efficient trafficking of TGFalpha to the cell;surface in polarized epithelial (C) 2004 Elsevier Inc. All rights reserved.