Modular organization of the PDZ domains in the human discs-large protein suggests a mechanism for coupling PDZ domain-binding proteins to ATP and the membrane cytoskeleton

The human homologue (hDlg) of the Drosophila discs-large tumor suppressor (Dlg) is a multidomain protein consisting of a carboxyl-terminal guanylate kinase-like domain, an SH3 domain, and three slightly divergent copies of the PDZ (DHR/GLGF) domain. Here we have examined the structural organization of the three PDZ domains of hDlg using a combination of protease digestion and in vitro binding measurements. Our results show that the PDZ domains are organized into two conformationally stable modules, one (PDZ(1+2)) consisting of PDZ domains 1 and 2, and the other (PDZ(3)) corresponding to the third PDZ domain. Using amino acid sequencing and mass spectrometry, we determined the boundaries of the PDZ domains after digestion with endoproteinase Asp-N, trypsin, and alpha-chymotrypsin. The purified PDZ(1+2), but not the PDZ(3) domain, contains a high affinity binding site for the cytoplasmic domain of Shaker-type K+ channels. Similarly, we demonstrate that the PDZ(1+2) domain can also specifically bind to ATP. Furthermore, we provide evidence for an in vivo interaction between hDlg and protein 4.1 and show that the hDlg protein contains a single high affinity protein 4.1-binding site that is not located within the PDZ domains. The results suggest a mechanism by which PDZ domain-binding proteins may be coupled to ATP and the membrane cytoskeleton via hDlg.