2 INDEPENDENT DOMAINS OF HDLG ARE SUFFICIENT FOR SUBCELLULAR TARGETING - THE PDZ1-2 CONFORMATIONAL UNIT AND AN ALTERNATIVELY SPLICED DOMAIN

hDlg, a human homologue of the Drosophila Dig tumor suppressor, contai ns two binding sites for protein 4.1, one within a domain containing t hree PSD-95/Dlg/ZO-1 (PDZ) repeats and another within the alternativel y spliced I3 domain. Here, we further define the PDZ-protein 4.1 inter action in vitro and show the functional role of both 4.1 binding sites in situ. A single protease-resistant structure formed by the entirety of both PDZ repeats 1 and 2 (PDZ1-2) contains the protein 4.1-binding site. Both this PDZ1-2 site and the I3 domain associate with a 30-kD NH2-terminal domain of protein 4.1 that is conserved in ezrin/radixin/ moesin (ERM) proteins. We show that both protein 4.1 and the ezrin ERM protein interact with the murine form of hDlg in a coprecipitating im mune complex. In permeabilized cells and tissues, either the PDZ1-2 do main or the I3 domain alone are sufficient for proper subcellular targ eting of exogenous hDlg. In situ, PDZ1-2-mediated targeting involves i nteractions with both 4.1/ERM proteins and proteins containing the COO H-terminal T/SXV motif. I3-mediated targeting depends exclusively on i nteractions with 4.1/ERM proteins. Our data elucidates the multivalent nature of membrane-associated guanylate kinase homologue (MAGUK) targ eting, thus beginning to define those protein interactions that are cr itical in MAGUK function.