A, shown schematically is the modular arrangement of the MAGUK ZO-1. The unique regions 5 and 6 are labeled U5 and U6, respectively. B, a ribbon diagram of a ZO-1 PDZ2 monomer in blue with the secondary structure labeled. The N and C termini are marked with N and C, respectively. C, stereo view of a ZO-1 PDZ2 domain-swapped dimer. One monomer is depicted in blue and the other in red.

A, overlay of ZO-1 PDZ2 and ZO-1 PDZ1 (Protein Data Bank code 2H2B). One monomer of the PDZ2 dimer is depicted in blue and the other in red. The ZO-1 PDZ1 domain is in green. The ligand bound to the ZO-1 PDZ1 domain is depicted as a yellow strand. The overlay shows a good fit between the PDZ1 and PDZ2 domains, albeit in the case of PDZ2, the structural elements are contributed by both molecules. Arrow 1 points at strand β1, and arrow 2 at strand β2. Importantly, despite the domain swapping that occurs in ZO-1 PDZ2, the peptide-binding grove of this module is not affected. The region where domain swapping commences is magnified. B and C, topology diagrams of ZO-1 PDZ2 and ZO-1 PDZ1, respectively. Color scheme as in A. D, amino acid sequence alignment of the N-terminal region of PDZ domains from ZO-1, -2, and -3 PDZ2; APBA1-PDZ2 (myeloid β A4 precursor protein-binding family A member 1); SDCB1- PDZ2 (syntenin-1); SIPA1-PDZ (signal-induced proliferation-associated protein 1); ZO-1-PDZ3; GRIP1-PDZ6; ZO-1-PDZ1; and Shank1-PDZ (SH3 and multiple ankyrin repeat domains protein 1). All sequences are of the human proteins and were aligned using ClustalW. The secondary structural elements for ZO-1 PDZ2 and PDZ1 are shown above their respective sequences. The secondary structure of APBA1, SDCB1, GRIP1, and Shank1 PDZ domains resemble that of ZO-1 PDZ1. DS-dimer, domain-swapped dimer.

A, a detailed representation of the three regions of interaction. On the bottom right, the interactions between β1 of one monomer and β6 of the other are shown. Likewise, interactions are shown on the bottom left, between a section of β2 and β3 and top left, between β2 of one monomer and its symmetrical segment from the second monomer. Only half of the total interactions per dimer are shown; a corresponding half is formed in the other part of the dimer. B, hydrophobic side chains from strands β1 and β2 of one molecule blue) are buried by residues from the complementary molecule (red). For clarity only the relevant parts of each molecule are displayed. Two views related by a 90° rotation are shown.

A, the ZO-1 PDZ2 dimer is shown with one monomer in dark blue and the other in light blue. The strands mediating dimerization are in yellow and red. Below the structure figure is a schematic of the interactions (displayed by dotted lines) between the monomer in the dimer. B, GRIP1-PDZ6 (Protein Data Bank code 1N7F) also forms a homodimer (dark and light red). However, here the interactions are predominately provided by residues from β1, with only additional few from the loop connecting α1 with β4. A schematic as in A is below the structure. C, dimerization of Shank1 PDZ (Protein Data Bank code 1Q3P) also depends on β1, with minor contribution from the loop connecting β2 and β3. A schematic as in A is below the structure. D, overlay of the three dimers done by superpositioning one of the monomers in each dimer (region used for superposition is boxed). Although the overall fold of the PDZ domains is nearly identical, due to different dimerization motifs, the orientation of the molecule completing the dimer is very different among the three homodimers.

The connexin-PDZ2 interface involves both the canonical PDZ interaction plus an interaction supplied by more C-terminal residues in connexins. A, sequence alignment of approximately 20 C-terminal residues of human Cx40, Cx43, Cx45, and Cx47. Residues involved in the canonical PDZ interaction are shaded gray. Pronounced homology is observed N-terminal to this sequence, shaded black. B, a model of Cx43 binding to PDZ2. PDZ2 is represented as a space-filling model, with one monomer in blue and the other in red. The most six terminal residues of Cx43 (yellow coil and strand) were modeled based on the binding of peptide to the ZO-1 PDZ1 domain. Note how the Cx43 chain is predicted to extend from one PDZ molecule to the next, where the conserved ASSR/K sequence could be important for providing additional binding elements to the ZO-1 PDZ2 domain.