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

The cytoplasmic domain of the transforming growth factor-α precursor (proTGFα) 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 TGFα's TVV. MAGI-3 is widely expressed in multiple mouse tissues, including brain. Immunolocalization showed that MAGI-3 and TGFα were colocalized in neurons in the cortex and dentate gyrus, as well as in ependymal cells and some astrocytes. In vitro, proTGFα 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 proTGFα interact transiently in MDCK cells stably transfected with both MAGI-3 and human proTGFα cDNAs. MAGI-3 and wild-type proTGFα colocalize at the cell surface. In contrast, MAGI-3 forms a stable complex with membrane-fixed TGFα early in the secretory pathway and interacts with immature and cell surface forms of membrane-fixed TGFα. Overexpression of MAGI-3 resulted in increased levels of TGFα in the basolateral medium of polarized MDCK cells, suggesting that MAGI-3 has a role in efficient trafficking of TGFα to the cell surface in polarized epithelial cells.