Figure 2. Innovations in Cadherin-based Adhesion and Junctions Across Species.

Some single celled organisms closely related to metazoans contain components similar to those found in adherens junctions, but there is no evidence that they interact with each other or mediate the formation of cell-cell junctions. For example, choanoflagellates have non-classical cadherins, which lack the intracellular domains able to interact with catenins. Porifera (sponges), metazoans distantly related to vertebrates, contain the major components of adherens junctions and there is evidence in some cases that these components interact to form junctional complexes. In addition, these cell-cell adhesive junctions can contain focal adhesion proteins including homologs for focal adhesion kinase, a β-integrin and vinculin. Only some porifera have a basement membrane, but even when absent focal adhesions can be present at the cell-substrate interface, which can contain β-catenin. Thus, the appearance of components of adherens junctions was an early event in metazoan evolution. Linking intermediate filaments to membrane junctions occurred later, for instance in C. elegans, where hemidesmosome-like structures in epithelial cells link intermediate filaments to the extracellular matrix through interactions between the spectraplakin VAB-10a and the transmembrane protein myotactin. There are two known cell-cell junction complexes in C. elegans epithelia: the most apical are vertebrate adherens junction homologs, and the basal are DLG-1/AJM-1 complex (DAC) junctions. Desmosomes appear in vertebrates concomitant with the appearance of desmosomal cadherins and the plakin protein desmoplakin, which links cell-cell junctions to intermediate filaments. In vertebrate simple epithelia, desmosomes are one of three intercellular junctions, which also include more apical tight junctions and adherens junctions. An expansion in the number of desmosomal cadherins occurred in mammals, where additional desmosomal cadherins are expressed in distinct patterns across different cell layers of complex epithelia. *Mixed adhesive junctions in porifera can contain adherens junction and focal adhesion proteins. **Focal adhesions in porifera can contain β-catenin.