Family-wide investigation of PDZ domain-mediated protein-protein interactions implicates β-catenin in maintaining the integrity of tight junctions

Abstract

β-catenin is a multifunctional protein that plays a critical role in cell-cell contacts and signal transduction. β-catenin has previously been shown to interact with PDZ-domain-containing proteins through its C terminus. Using protein microarrays comprising 206 mouse PDZ domains, we identified 26 PDZ-domain-mediated interactions with β-catenin and confirmed them biochemically and in cellular lysates. Many of the previously unreported interactions involved proteins with annotated roles in tight junctions. We found that four tight-junction-associated PDZ proteins-Scrib, Magi-1, Pard3, and ZO-3-colocalize with β-catenin at the plasma membrane. Disrupting these interactions by RNA interference, overexpression of PDZ domains, or overexpression of the β-catenin C terminus altered localization of the full-length proteins, weakened tight junctions, and decreased cellular adhesion. These results suggest that β-catenin serves as a scaffold to establish the location and function of tight-junction-associated proteins.

Figure 1. Identification of β-catenin–PDZ domain interactions using protein microarrays and fluorescence polarization

(A) Images of PDZ domain microarrays, probed with a fluorescently labeled peptide derived from the C-terminus of β-catenin. The Cy5 image (green) shows the placement of the spots on one array, and the 5(6)-TAMRA images (red) show binding of the peptide to immobilized PDZ domains. The PDZ domains were spotted in duplicate from top to bottom as shown. Interactions with PDZ domains derived from Magi-1, Scrib, Pard3 and Mupp1 are highlighted in white boxes. (B) Confirmation and quantification of β-catenin–PDZ domain interactions by fluorescence polarization. Saturation binding curves are shown for PDZ domains derived from Magi-1, Scrib, Pard3, and Mupp1. See also Figure S1, Table S1.

Figure 2. Validation of β-catenin–PDZ domain interactions by His₆ pull-down and Co-IP assays

(A) Western blots showing that full-length β-catenin from HEK 293 cells binds to purified PDZ domains in a His₆ pull-down assay. Noncognate PDZ domains serve as negative controls. The numbers in parentheses refer to the PDZ domain number (counting from the N-terminus) and the total number of PDZ domains in the full-length sequence. (B) Western blot showing expression of full-length Myc-tagged β-catenin (top) and full-length HA-tagged PDZ domain-containing proteins (bottom) in whole cell lysates derived from transiently transfected HEK 293 cells. (C, D) Full-length β-catenin associates with full-length HA-tagged PDZ domain-containing proteins. HEK 293 cells were transiently co-transfected with Myc-tagged β-catenin and full-length HA-tagged PDZ domain-containing proteins. Myc-tagged β-catenin was immunoprecipitated with an anti-Myc antibody and co-purifying proteins were detected by immunoblotting with an anti-HA antibody. (C) Previously reported interactions between β-catenin and both Dvl3 and Mals2 were confirmed. (D) Novel interactions were observed between β-catenin and Scrib, Tiam2, Grasp55, Omp25, Mast1 and Pdzk7. Data shown are representative images from at least three independent experiments. See also Figure S1.

Figure 3. β-catenin interacts with several tight junction-associated PDZ domain-containing proteins

(A) Pie chart showing the cellular component classification as reported in the Gene Ontology database (http://www.geneontology.org) of all PDZ domain-containing proteins that were identified as binding partners of β-catenin. PDZ domain-containing proteins that are known to localize in tight junctions are highlighted in green. (B) Domain structures of Magi-1, Pard3, Scrib, and ZO-3. (C) Investigation of which PDZ domains in Magi-1, Pard3, Scrib, and ZO-3 bind full-length endogenous β-catenin in a co-IP assay. HEK 293 cells were transiently transfected with Myc-tagged versions of isolated PDZ domains or PDZ domain clusters derived from Magi-1, Pard3, Scrib, and ZO-3. PDZ domains were immunoprecipitated using an anti-Myc antibody and interactions were detected by immunoblotting for β-catenin. Whole cell lysates showing expression of the Myc-tagged PDZ domains and endogenous β-catenin are also displayed. Data shown are representative images from at least three independent experiments.

Figure 4. Localization of β-catenin and tight junction-associated PDZ proteins in epithelial cells

(A) Confocal images showing that β-catenin (red) co-localizes with the PDZ domain-containing tight junction proteins Magi1, Pard3, Scrib, and ZO-3 (green) in MDCK kidney epithelial cells. (B) In SW480 colon cancer cells, β-catenin (red) is mostly localized in the cytoplasm and nucleus, whereas the PDZ domain-containing tight junction proteins Magi1, Pard3, Scrib, and ZO-3 are localized in the cytoplasm. The merged images showing both β-catenin and the PDZ domain-containing tight junction proteins are displayed to the right. Yellow indicates co-localization. Nuclei are stained in blue (Hoechst dye). Data shown are representative images from at least two independent experiments. (C) Confocal images showing β-catenin (red) co-localizes with the PDZ domain-containing tight junction protein Scrib (green) in Caco-2 epithelial cells (top). Knocking down the levels of β-catenin results in decreased membranous expression of endogenous Scrib (middle) whereas knocking down Scrib did not affect the membranous localization of β-catenin (bottom). Scale bars are 10 μm. See also Figures S2 and S3.

Figure 5. Overexpression of β-catenin-interacting PDZ domains affects the strength of tight junctions

(A) Confocal images showing membranous localization of endogenous Scrib MDCK cells (i) and cytosolic localization of Myc-tagged PDZ3 of Scrib (ii, v, and viii). MDCK cells expressing PDZ3 of Scrib also showed altered expression of the tight junction marker occludin (iii, iv) and endogenous Scrib (vi, vii). The white arrows indicate localization of occludin in cells that do not express Myc-tagged PDZ3 of Scrib or PDZ6 of Magi-1; yellow arrows indicate occludin localization in cells expressing PDZ3 of Scrib. The membranous localization of endogenous β-catenin in cells expressing PDZ3 of Scrib is also shown (ix, x). Blue indicates nuclei (Hoechst stain); yellow indicates co-localization. Scale bars are 10 μm. (B) MDCK cells expressing PDZ4 of Scrib (i, iv, vii) do not show altered expression or localization of either occludin (ii, iii) or endogenous Scrib (v, vi). The membranous localization of endogenous β-catenin in cells expressing PDZ4 of Scrib is also shown (viii, ix). (C) Quantification of occludin localization in parental MDCK cells or in cells expressing isolated PDZ domains derived from tight junction-associated proteins. Occludin length was measured using ImageJ (http://rsbweb.nih.gov/ij/). Error bars indicate the SEM of at least four measurements. (D) Transepithelial electrical resistance (TEER), measured in parental MDCK cells and in cells expressing individual PDZ domains. TEER values are the mean of at least three biological replicates, error bars represent the SEM, and asterisks denote a significant difference from mock-transfected parental cells (P < 0.05). (E) Ca²⁺-switch assay was performed with parental or MDCK cells expressing Myc-Scrib (PDZ3) and Myc-Scrib (PDZ4) constructs. Cell layers were fixed and stained for occludin and Myc-tag. Confocal images of occludin (green) and Myc tag-scrib PDZ domain (red) are shown. (F) Graphs show the results of quantification of occludin recruitment to cell-cell contacts in parental or MDCK cells overexpressing isolated PDZ domains. (G) Relative adhesion of parental MDCK cells and cells expressing isolated PDZ domains grown on Fibronectin, Collagen I, and Laminin I. Cell adhesion was measured 48 h post-transfection. Adhesion values are the means of at least three biological replicates, error bars represent the SEM, and asterisks denote a significant difference from mock-transfected parental cells (P < 0.05). (H) Relative proliferation of parental MDCK cells and cells transfected with isolated PDZ domains. Each proliferation value is the mean of at least three biological replicates, error bars represent the SEM, and asterisks denote a significant difference from mock-transfected parental cells (P < 0.05). See also Figures S4 and S5.

Figure 6. Overexpressing the PDZ domain-binding motif of β-catenin affects tight junctions

(A) Schematic showing the carboxyl terminus of β-catenin (GFP-βcat-Cterm: NH2-GWFDTDL-COOH) and GFP-mut-Cterm, a nonbinding mutant version in which the last three residues were mutated to alanine (NH2-GWFDAAA-COOH). (B) Confocal images showing MDCK cells expressing either GFP-βcat-Cterm or GFP-mut-Cterm and endogenous subcellular localization of Scrib. GFP-βcat-Cterm and endogenous Scrib were observed to co-localize in the cytosol (yellow); GFP-mut-Cterm, on the other hand, had no appreciable affect on the subcellular localization of Scrib and did not co-localize with endogenous Scrib. (C) Bar graph showing quantification of occludin length in a Ca²⁺-switch assay. Overexpressing GFP-βcat-Cterm caused a significant decrease in the average length of occludin per cell, whereas overexpressing GFP-mut-Cterm had no effect (P < 0.05). (D) TEM images of MDCK cells overexpressing GFP-mut-Cterm showed the presence of typical tight junctions (yellow arrow), as well as adherens junctions (white arrow), in sites of cell-cell contact. In contrast, cells overexpressing GFP-βcat-Cterm showed weaker or no tight junctions. (E) MDCK cells expressing GFP-βcat-Cterm also exhibited a decreased ability to attach to extracellular matrices. When grown on Fibronectin, Collagen I, or Fibrinogen, MDCK cells overexpressing GFP-βcat-Cterm exhibited significantly less adhesion, whereas MDCK cells expressing GFP-mut-Cterm showed little or no change in their adhesion properties relative to untransfected cells. (F, G) Overexpressing GFP-βcat-Cterm caused significant increases in migration and invasion (p<0.05); no significant change was observed with the mutant control. Each bar is the mean of at least three biological replicates, error bars represent the SEM, and asterisks denote a significant difference from mock-transfected parental cells (P < 0.05).