Ann. N.Y. Acad. Sci. ISSN 0077-8923 ANNALS OF THE NEW YORK ACADEMY OF SCIENCES Issue: Barriers and Channels Formed by Tight Junction Proteins ZO-2, a tight junction scaffold protein involved in the regulation of cell proliferation and apoptosis Lorenza Gonzalez-Mariscal, Pablo Bautista, Susana Lechuga, and Miguel Quiros Center of Research and Advanced Studies (Cinvestav), Department of Physiology, Biophysics and Neuroscience, Mexico D.F., Mexico Address for correspondence: Lorenza Gonzalez-Mariscal, Ph.D., Center of Research and Advanced Studies (Cinvestav), Department of Physiology, Biophysics, and Neuroscience, Ave. IPN 2508, Mexico, D.F. 07360, Mexico. lorenza@fisio.cinvestav.mx ZO-2 is a membrane-associated guanylate kinase homologue (MAGUK) tight protein associated with the cytoplasmic surface of tight junctions. Here, we describe how ZO-2 is a multidomain molecule that binds to a variety of cell signaling proteins, to the actin cytoskeleton, and to gap, tight, and adherens junction proteins. In sparse cultures, ZO-2 is present at the nucleus and associates with molecules active in gene transcription and pre-mRNA processing. ZO-2 inhibits the Wnt signaling pathway, reduces cell proliferation, and promotes apoptosis; its absence, mutation, or overexpression is present in various human diseases, including deafness and cancer. Keywords: tight junctions; ZO-2; scaffold; cell proliferation; apoptosis Introduction ZO is an acronym of zonula occludens, the Latin name for tight junctions. ZO-2 is a peripheral pro- tein associated with the cytoplasmic surface of tight junctions, originally identified in 1991 in a ZO-1 immunoprecipitate. 1 Here, we describe how ZO-2 is a multidomain scaffold that at the cell membrane binds to gap, tight, and adherens proteins and at the nucleus associates with molecules active in gene transcription and pre-mRNA processing. Then, we will detail how ZO-2 inhibits the Wnt signaling pathway, reduces cell proliferation, and promotes apoptosis, and will relate these observations to hu- man disease. Molecular structure and protein–protein interactions ZO-2 is a 160 kDa scaffold molecule with several different protein-binding domains (Fig. 1). ZO- 2 belongs to the MAGUK (membrane-associated guanylate kinase homologue) family as it has three PDZ domains, one SH3 module, and a GuK do- main. In addition, the carboxyl segment of ZO-2 has acidic and proline-rich domains. Between these domains, ZO-2 exhibits linker regions also named unique (U). At the carboxyl terminal, the last three amino acids, TEL, constitute a PDZ-binding motif. The first PDZ domain of ZO-2 associates with the scaffold attachment factor SAF-B, 2 a DNA-binding protein that serves as a molecular base to assemble a transcriptosome complex in the vicinity of actively transcribed genes, and to the PDZ-binding motifs of: the tight junction integral proteins claudins 1 to 8; 3,4 the adenovirus type 9 oncogenic E4-ORF1 protein; 5 and the transcriptional coactivator TAZ implicated in cell proliferation and epithelial to mesenchymal transition 6 and of its paralog YAP that promotes cell detachment and apoptosis. 7 The second PDZ domain of ZO proteins is re- sponsible for the homo and hetero dimerization of these molecules. 8,9 The crystal structure of the sec- ond PDZ domain of human ZO-2 reveals that homo dimerization takes place by domain swapping of 1 and 2 strands. 10 In addition, NMR spectroscopy has demonstrated the formation of ZO-1-PDZ- 2/ZO-2-PDZ-2 heterodimers by domain swapping, owing to the 68% identity present between the PDZ-2 domains of these proteins. 11 ZO-2-PDZ-2 homodimers have a similar binding pattern to that exhibited in the ZO-1-PDZ-2/connexin 43 com- plex, 10 and interestingly, ZO-2-PDZ-2 additionally doi: 10.1111/j.1749-6632.2012.06537.x Ann. N.Y. Acad. Sci. 1257 (2012) 133–141 c ⃝ 2012 New York Academy of Sciences. 133