Occludin is an integral membrane phosphoprotein specifically associated with tight junctions, contributing to the structure and function of this intercellular seal. Occludin function is thought to be regulated by phosphorylation, but no information is available on the molecular pathways involved. In the present study, the involvement of the protein kinase C pathway in the regulation of the phosphorylation and cellular distribution of occludin has been investigated. Phorbol 12-myristate 13-acetate and 1,2-dioctanoylglycerol induced the rapid phosphorylation of occludin in Madin-Darby canine kidney cells cultured in low extracellular calcium medium with a concomitant translocation of occludin to the regions of cell-cell contact. The extent of occludin phosphorylation as well as its incorporation into tight junctions induced by protein kinase C activators or calcium switch were markedly decreased by the protein kinase C inhibitor GF-109203X. In addition, in vitro experiments showed that the recombinant COOH-terminal domain of murine occludin could be phosphorylated by purified protein kinase C. Ser 338 of occludin was identified as an in vitro protein kinase C phosphorylation site using peptide mass fingerprint analysis and electrospray ionization tandem mass spectroscopy. These findings indicate that protein kinase C is involved in the regulation of occludin function at tight junctions.

The exact sites, structures, and molecular mechanisms of interaction between junction organizing zona occludence protein 1 (ZO-1) and the tight junction protein occludin or the adherens junction protein ␣-catenin are unknown. Binding studies by surface plasmon resonance spectroscopy and peptide mapping combined with comparative modeling utilizing crystal structures led for the first time to a molecular model revealing the binding of both occludin and ␣-catenin to the same binding site in ZO-1. Our data support a concept that ZO-1 successively associates with ␣-catenin at the adherens junction and occludin at the tight junction. Strong spatial evidence indicates that the occludin C-terminal coiled-coil domain dimerizes and interacts finally as a four-helix bundle with the identified structural motifs in ZO-1. The helix bundle of occludin 406 -521 and ␣-catenin Different junctional complexes such as adherens junctions and, in specialized tissues, tight junctions, gap junctions, and desmosomes connect cells in multicellular organisms. TJ 1 seal the most apical-lateral parts of cells and constitute a diffusion barrier for the paracellular flow of molecules and serve as a fence between the apical and basolateral membrane compartment (1). In contrast, AJ play important roles in cell adhesion, migration, morphogenesis, and proliferation. AJ represent Ca 2ϩ -dependent cell-cell contacts localized basolateral of TJ, where transmembrane proteins of the cadherin family mediate adhesion. Assembly of AJ is a prerequisite for the formation of TJ and desmosomes (2). ␤-and ␣-catenin bind to the cadherin cytoplasmic domain and link the cadherin-catenin complex to the F-actin cytoskeleton. An important scaffolding protein in cell-cell contacts is the zona occludens protein 1. ZO-1 is a membrane associated guanylate kinase homologue protein composed of the following domains: three PDZ (PSD95/Dlg/ZO-1), a SH3, a GuK (3), an actin binding region (4), and a ZU5 (ZO-1 and Unc5-like netrin receptor domain) according SMART (Simple Modular Architecture Research Tool (smart.embl-heidelberg.de) data base (5)). In non-epithelial cells ZO-1 is a major component of AJ, whereas in epithelial cells it is localized at TJ by directly binding to claudins (6). Occludin, one of the transmembrane proteins of TJ, is a multiphosphoprotein involved in regulation of TJ (7). It has four transmembrane domains with two extracellular loops and a cytosolic N and C terminus. A sequence of 244 amino acids in human ZO-1 containing the GuK domain and an acidic region C terminus to GuK binds to the complete intracellular C-terminal tail of chicken occludin (8).ZO-1 also binds to the AJ protein ␣-catenin (9) and to connexins in gap junctions (10), indicating a general scaffolding function of ZO-1 in junctional complexes. ␣-Catenin consists of several four-helix-bundle domains (vinculin homology domains, VH1-3) and binds ␤-catenin via an intermolecular helix bundle mechanisms within the E-cadherin-catenin complex at the intracellular side of AJ, where one helix of ...

The proper cellular location and sealing of tight junctions is assumed to depend on scaffolding properties of ZO-1, a member of the MAGUK protein family. ZO-1 contains a conserved SH3-GUK module that is separated by a variable region (unique-5), which in other MAGUKs has proven regulatory functions. To identify motifs in ZO-1 critical for its putative scaffolding functions, we focused on the SH3-GUK module including unique-5 (U5) and unique-6 (U6), a motif immediately C-terminal of the GUK domain. In vitro binding studies reveal U5 is sufficient for occludin binding; U6 reduces the affinity of this binding. In cultured cells, U5 is required for targeting ZO-1 to tight junctions and removal of U6 results in ectopically displaced junction strands containing the modified ZO-1, occludin, and claudin on the lateral cell membrane. These results provide evidence that ZO-1 can control the location of tight junction transmembrane proteins and reveals complex protein binding and targeting signals within its SH3-U5-GUK-U6 region. We review these findings in the context of regulated scaffolding functions of other MAGUK proteins. INTRODUCTIONTight junctions (TJ) are a hallmark of polarized epithelial cells, providing the paracellular barrier required to separate tissue spaces (Van Itallie and Anderson, 2004), contributing to maintenance of apical-basolateral cell polarity and providing a site for cell-cell signaling (Schneeberger and Lynch, 2004;Matter et al., 2005;Shin et al., 2006). At the ultrastructural level, tight junctions appear as highly organized strands that encircle the apical-lateral boundary. These strands are composed of transmembrane proteins such as occludin, tricellulin, and one or more members of the claudin family, which create the paracellular barrier. These proteins are, in turn, associated with a cytosolic plaque of proteins that is closely associated with the cortical cytoskeleton. The sequence of events that bring about the proper continuous apical localization of TJ proteins remains unknown. One approach to this problem has been to define components in upstream pathways required to induce cell polarity (Shin et al., 2006). Another approach has been to elaborate the biochemical interactions among the more than 40 transmembrane and cytosolic components of the TJ and to define their functional role in somatic cells using gene ablation, transcriptional inhibition, or dominant negative strategies (Schneeberger et al., 2004). Despite considerable efforts by many groups, the precise mechanism of junction assembly is poorly understood.The cytoplasmic protein ZO-1 is proposed to be one of the key regulators of TJ assembly (reviewed in Fanning, 2006). ZO-1 is member of a large family of membrane-associated scaffolding and signaling molecules known as the membrane-associated guanylate kinase homologues (MAGUKs). These proteins are characterized by a core motif of conserved protein-binding domains including one or more PDZ domains, an SH3 domain and a GUK domain (Funke et al., 2005). By analogy to other MAGUKs,...

Edited by Ruma Banerjee Loss-of-function mutations in the gene encoding human protein DJ-1 cause early onset of Parkinson's disease, suggesting that DJ-1 protects dopaminergic neurons. The molecular mechanisms underlying this neuroprotection are unclear; however, DJ-1 has been suggested to be a GSH-independent glyoxalase that detoxifies methylglyoxal (MGO) by converting it into lactate. It has also been suggested that DJ-1 serves as a deglycase that catalyzes hydrolysis of hemithioacetals and hemiaminals formed by reactions of MGO with the thiol and amino groups of proteins. In this report, we demonstrate that the equilibrium constant of reaction of MGO with thiols is ϳ500 M ؊1 at 37°C and that the half-life of the resulting hemithioacetal is only 12 s. These thermodynamic parameters would dictate that a significant fraction of free MGO will be present in a fast equilibrium with hemithioacetals in solution. We found that removal of free MGO by DJ-1's glyoxalase activity forces immediate spontaneous decomposition of hemithioacetals due to the shift in equilibrium position. This spontaneous decomposition of hemithioacetals could be mistaken for deglycase activity of DJ-1. Furthermore, we demonstrate that higher initial concentrations of hemithioacetals are associated with lower rates of DJ-1mediated conversion of MGO, ruling out the possibility that hemithioacetals are DJ-1 substrates. Experiments with CRISPR/ Cas-generated DJ-1-knockout HEK293 cells revealed that DJ-1 does not protect against acute MGO toxicity or carboxymethylation of lysine residues in cells. Combined, our results suggest that DJ-1 does not possess protein deglycase activity.

The tight junction protein ZO-1 is known to link the transmembrane proteins occludin, claudins, and JAMs to many cytoplasmic proteins and the actin cytoskeleton. Although specific roles for ZO-1 at the tight junction are unknown, it is widely assumed that ZO-1, together with its homologs ZO-2 and ZO-3, serves as a platform to scaffold various transmembrane and cytoplasmic tight junction proteins. Thus the manner in which the zonula occludens (ZO) proteins multimerize has implications for the protein networks they can coordinate. The purpose of our study was to determine whether ZO-1 forms homodimers and to determine the protein interaction region. Using laser light scattering and analytical centrifugation, we show that protein sequences corresponding to the NH 2 -terminal half of ZO-1 form stable homodimers with a submicromolar equilibrium dissociation constant. Analysis of the molecular weight of different truncated forms of ZO-1 revealed that the second PDZ domain is both necessary and sufficient for dimerization. This interaction does not use the ␤-finger motif described for other PDZ dimers. Furthermore, ZO-1 does not dimerize via an Src homology 3 to Guk domain interaction as was demonstrated previously for MAGUKs, like PSD-95. Results from immunoprecipitation experiments with polarized Madin-Darby canine kidney epithelial cells stably transfected with full-length GFP-ZO-1 indicate that a substantial portion of ZO-1 forms homodimers in vivo. As described previously, ZO-1 also forms heterodimers with ZO-2 and ZO-3. We conclude that the dimerization of ZO proteins is unlike that of other MAGUKs and that the previously unrecognized ZO-1 homodimers may allow formation of protein networks distinct from those of heterodimers with ZO-2 and ZO-3. The tight junction (TJ)2 is the apical-most element of the junctional complex in epithelial and endothelial cells. This complex includes the TJ, adherens junctions, desmosomes, and gap junctions. The TJ forms a major barrier to paracellular movement of substances preserving unique chemical composition of apical and basolateral fluid compartments on opposite sides of the epithelial cell layer and coordinates transmembrane signaling (1, 2). The TJ is known to contain at least four classes of transmembrane proteins and Ͼ30 cytoplasmic signaling, scaffolding, and cytoskeletal proteins (3). To understand how the barrier and signals are coordinated, more information is required on the specific molecular interactions of TJ proteins and specifically the proteins that coordinate interaction among transmembrane and cytosolic proteins.Peripheral membrane proteins of the tight junction are believed to be highly organized within a complicated network interconnecting cytoskeleton with transmembrane proteins. The three homologous zonula occludens (ZO) MAGUK proteins (ZO-1, ZO-2, and ZO-3) are among the best characterized peripheral membrane proteins of the tight junction. They are composed of three PDZ domains, an SH3-GUK unit, and an actin-binding region; these proteins are believed to ...