Unraveling the cytoplasmic interactions of the cadherin superfamily

Cadherins are a rapidly growing family of calcium-dependent morphoregulatory proteins. Members of three subsets of this family-classical cadherins, desmocollins, and desmogleins-are the transmembrane components of adherens junctions and desmosomes, respectively, the major structures responsible for cellcell adhesion and cytoskeletal-membrane attachment (1, 2). Several other types of cadherin have recently been discovered, such as the protocadherins and the products of the, Drosophila genes Fat and Dachsous (3, 4). These proteins are less well defined in terms of their cellular location and function, but in some cases are known to profoundly affect cellular differentiation and display tumor suppressor properties. The most completely studied proteins within this family are the classical cadherins, such as Eand N-cadherin. These proteins, in addition to maintaining the coherence of adult tissues through their role in adherens junctions, are important during development (5-7). They display intricate temporal and spatial patterns of expression during early embryogenesis and mediate cellular recognition events through highly specific homophilic interactions (1). A number of experiments have shown that the expression of classical cadherins dramatically alters the morphology and behavior of cultured cells (8, 9). How cadherins evoke such changes has been the holy grail of the field during the last decade. Early studies defined a crucial role for the highly conserved carboxyl termini of classical cadherins in eliciting cellular responses to engagement of their ectodomains. Expression of E-cadherin in L-cell fibroblasts, which are normally bipolar and nonadherent, causes them to adopt an epithelial appearance and to adhere tightly to one another, forming colonies (8). These changes are accompanied by an association of the cadherin molecules with the filamentous cytoskeleton and a reorganization and "polarization" of the subcortical cytoskeleton and several transmembrane components (10, 11). In contrast, cadherins deleted in their carboxyl-terminal region reach the cell surface and can bind soluble cadherin molecules but fail to associate with or polarize the subcortical and filamentous cytoskeleton and are unable to sustain adhesion between neighboring cells (12, 13). This carboxyl-terminal region is required for the association of classical cadherins with three proteins, a-catenin, ,Bcatenin, and plakoglobin (y-catenin) (14, 15). On the basis of the experiments described above, catenins and plakoglobin have been hypothesized to facilitate classical cadherin function, either by tethering these transmembrane molecules to the cytoskeleton thereby clustering them into junctional sites of high adhesive affinity and/or by relaying cadherin-contact-mediated signals (14, 15). Recently, their cDNAs have been cloned, permitting address of their precise functions and exact structural connections by molecular techniques (16-19). A series of recent articles, one of which is found in this issue of the Proceedings (20) are now beginning to solve some of the important structure-function questions concerning the catenins, plakoglobin, and otherjunction-associated molecules. An explosion of interest in such questions has been fueled by studies demonstrating that plakoglobin and /-catenin are structural and functional homologues of the product of a Drosophila segment polarity gene armadillo and furthermore that these same proteins associate with the product of a tumor suppressor gene (APC) linked to colon cancer (18, 21-28). Hence, catenins and plakoglobin are now viewed not only as facilitators of cadherinmediated cell adhesion but as central players in pathways involving the transmission of positional information and tumor suppression. The importance of a-catenin has been demonstrated in studies on the lung carcinoma cell line PC9 (29). Although these cells express normal levels of E-cadherin, they have no detectable levels of a-catenin, are poorly adhesive, and grow in suspension as loose aggregates. Transfection of these cells with cDNA encoding a-catenin promotes strong intercellular adhesion, causing them to form spherical cysts in culture. The sequence of a-catenin provides perhaps the strongest clues to its function (16, 17). It shares homology with three regions, including the self-association domain, of vinculin, a protein found at several sites of membrane attachment of actin filaments. Therefore, by analogy it is thought that a-catenin may provide a mechanism to oligomerize proteins to which it is bound and/or to attach them to the actin cytoskeleton. Recently Walter Birchmeier and colleagues (30) have shown that a-catenin binds indirectly to E-cadherin, as well as to APC protein (named for adenomatosis polyposis coli), through association with the N-terminal head domain of either plakoglobin or -catenin. These investigators have proposed that both E-cadherin and APC protein may exert their tumor suppressor properties by interacting with the cytoskeleton. Plakoglobin and ,-catenin show significant sequence similarity to each other as well as to the segment polarity gene armadillo (18, 19, 22, 31). Each of these proteins contains a central region of 13 repeats flanked by more variable head and tail regions. In addition to their structural resemblance, these proteins are clearly functional homologues. The armadillo protein takes part in a signaling pathway that establishes cell fate across the body segments of Drosophila. This protein shows a rapid posttranscriptional elevation in response to a secreted protein, wingless (32, 33). Plakoglobin and ,-catenin display a similar response in vertebrates to Wnt protein, a vertebrate homologue of wingless (24-26). Studies in cultured cells suggest that the response to Wnt entails a modulation in cell adhesion, via the well-established effect of plakoglobin and f3-catenin on cadherin function, which in turn facilitates communication between cells and causes reorganization of the cytoskeleton. Furthermore, some cells expressing Wnt display a modified cellular response to contact inhibition of growth, probably mediated through the cytosolic interactions of plakoglobin and f3-catenin (24, 25). ,3Catenin is found separately in complexes with E-cadherin and with APC protein (12, 27, 28). Plakoglobin has been found in mutually exclusive membrane-bound complexes with Eand N-cadherin, desmoglein, and desmocollin, as well as in cytosolic complexesboth as a homodimer and as a heterodimer with APC protein (20, 22, 29, 34-36). A growing body of evidence is compatible with the notion that at least