Mechanisms of Development, 44 (1993) 85-89 85 © 1993 Elsevier Scientific Publishers Ireland, Ltd. 0925-4773/93/$06.00 MOD 00205 ZO-1, DIgA and PSD-95/SAP90: homologous proteins in tight, septate and synaptic cell junctions 1 Daniel F. Woods * and Peter J. Bryant Developmental Biology Center, Universityof California, Irvine, CA 92717-2275, USA (Received 13 September 1993; accepted 22 September 1993) Epithelia of both vertebrateg and invertebrates pre- sent selective permeability barriers, maintaining differ- ent chemical compositions of fluids between the apical and basal sides of the cell layer. In vertebrates this barrier is maintained by the zonula occludens or tight junction (Fig. 1), which forms a continuous belt around the apical end of the epithelial cell and restricts the transepithelial (paracellular) diffusion of solutes, as assessed by the movement of various tracers (for review see Citi, 1993). Tight junctions also form an intramem- branous bari'ier that restricts the movement of mem- brane lipids and proteins between the apical and baso- lateral membrane domains, thus contributing to api- cal-basal cell polarity (for review see Citi, 1993). In invertebrates, tight junctions are usually missing, but instead there is a more elaborate structure, the septate junction, that is characterized by evenly spaced elec- tron-dense septa between cells (Fig. 1; Green and Bergquist, 1982). The septate junction appears to have a similar function to the tight junction in that it also forms a continuous belt around each cell, prevents transepithelial diffusion (Green and Bergquist, 1982) and restricts the movement of membrane lipids be- tween plasma membrane domains (Wood, 1990). How- ever, the fact that vertebrate tight junctions are apical to adherens junctions whereas invertebrate septate junctions are basal (Fig. 1) has always made the rela- tionship between these two junctional types puzzling. Here we show that one of the major components of the mammalian tight junction has a similar modular orga- nization and substantial sequence homology to a pro- tein of the insect septate junction. Thus, a striking molecular similarity between these two types of junc- tions reinforces the idea that they have similar func- tions. But more surprising is the presence of a homolo- * Corresponding author. 1GenBank accession numbers: PSD-95/SAP90, M96853/X66474; dig, M73529; ZO-1, D14340; GUK, $23776. gous protein in the synaptic junctions of mammalian brain. The dlg tumor suppressor gene of Drosophila en- codes a 960 amino-acid protein, DIgA (Figs. 2 and 3) that is localized to septate junctions in various epithe- lia (Woods and Bryant, 1991). The DlgA protein is also present at high levels in the nervous system, but its subcellular location in this tissue is not known. The predicted protein includes three copies of an ~ 80 amino-acid domain we call DHR (for Discs-large Ho- mologous Region, replacing the previous designation GLGF (Cho et al., 1992) since several homologs of this domain, including those in ZO-1, do not include the GLGF signature). It also includes a repetitive amino acid sequence derived from an OPA repeat similar to that found in a variety of other genes (Wharton et al., 1985), between DHR2 and DHR3. The gene product includes an SH3 domain (Musacchio et al., 1992), thought to mediate interactions with other proteins, a PEST region (Rogers et al., 1986) identified mainly by high content of specific amino acids and thought to be associated with rapid protein degradation, and a C- terminal ~ 180 amino-acid region with homology to guanylate kinase (GUK) of yeast (Stehle and Schulz, 1992) and pig (Zschocke et al., 1993). Although the GUK domain shows conservation or conservative sub- stitution of all the amino-acid residues that are known to interact with the GMP substrate, the A-motif (Fig. 3), thought to be part of the ATP-binding site (Koonin et al., 1992), is interrupted by a three amino-acid deficiency that might inactivate the domain as a guany- late kinase. Therefore the GUK domain might have a novel function requiring GMP binding. The major protein component of the mammalian tight junction, ZO-1 (Anderson et al., 1988; Itoh et al., 1993; Willott et al., 1993), shows clear homology to the DIgA protein in the three DHR domains, SH3, and GUK, but it differs from DIgA in the presence of a large proline-rich C-terminal extension (Figs. 2, 3). The GUK domain shows 29% identity of amino acid