Myristoylated Naked2 escorts transforming growth factor α to the basolateral plasma membrane of polarized epithelial cells

The epidermal growth factor receptor ligands transforming growth factor alpha (TGFalpha) and amphiregulin are delivered to the basolateral surface of polarized epithelial cells where they are cleaved by TACE/ ADAM17. Basolateral sorting information resides in their cytoplasmic tail domains, but tail-interacting proteins required for basolateral trafficking have not been identified. Naked (NKD)1 and NKD2 are mammalian homologs of Drosophila Naked Cuticle, which negatively regulates canonical Wnt signaling by binding Dishevelled. We present evidence that NKD2, but not NKD1, binds to basolateral sorting motifs in the cytoplasmic tail of TGFalpha. Processing and cell-surface delivery of TGFalpha are accelerated in NKD2-overexpressing Madin-Darby canine kidney cells. NKD2 is myristoylated on glycine, the second residue. On expression of myristoylation-defective (G2A) NKD2, neither NKD2 nor TGFa appears at the basolateral plasma membrane of polarized Madin-Darby canine kidney cells; however, membrane staining for TGFa is restored on silencing expression of this mutant NKD2. Amphiregulin does not interact with NKD2 and retains its basolateral localization in G2A-NKD2-expressing cells, as do Na+, K+ ATPase alpha1 and E-cadherin. These data identify an unexpected function for NKD2, i.e., myristoylation-dependent escort of TGFalpha to the basolateral plasma membrane of polarized epithelial cells.