The Unique Region Of A Desmosomal Cadherin, Desmoglein 2, Regulates Internalization Via Tail-Tail Interactions

Desmogleins and desmocollins are desmosomal cadherins that make up the adhesive core of desmosomes, intercellular junctions specialized for helping cells withstand mechanical stress. Desmoglein 2 (Dsg2) is the most widely expressed member of the Dsg subfamily and misregulation of Dsg2 has been reported in many types of cancer. While endocytosis is known to be a critical determinant in regulating cell surface levels of classic cadherins during tissue remodeling and cancer progression, less is known about the regulation of desmosomal cadherins. The trafficking of transmembane proteins is frequently dictated by the presence of specific sequences in their cytoplasmic tails. The cytoplasmic tail of Dsg contains a membrane proximal region, which is conserved in classic cadherins, and a desmoglein unique region (DUR), which is only present in the Dsg family. To determine the contribution of the Dsg2 tail to its endocytosis, we generated a series of mutants with progressive truncations of the cytoplasmic tail. A time course to track the internalized pool of Dsg2 mutants following cell surface biotinylation demonstrated that the DUR attenuates Dsg2 internalization. Analysis of a series of chimeric molecules in which variable lengths of the Dsg2 tail were fused with interleukin-2 ecto- and transmembrane domains showed that this function of the DUR is independent of the status of the Dsg2 ectodomain. With or without the DUR, internalization of the Dsg2 mutants exhibited the same dependence on cholesterol and dynamin, suggesting that the DUR inhibits Dsg29s entry into a common pathway(s). Detergent solubility and sucrose gradient fractionation assays indicated that the presence of the DUR alters the Dsg2 protein complex profile, even though there was no change in the interaction of Dsg2 mutants with known desmoglein binding partners, plakoglobin and plakophilin 3. To test whether the DUR controls the formation of Dsg2 complexes by affecting the oligomeric status of the Dsg2 cytoplasmic domain, we carried out in situ proximity ligation and yeast two hybrid analyses. The data suggest that Dsg2 inter- but not intra-molecular interactions occur, and that these are mediated through the DUR. To investigate whether DUR-dependent inhibition of internalization and mediation of tail-tail interactions are mechanistically linked, one copy of a dimerization motif was fused to the C-terminus of a Dsg2 construct lacking the DUR. Forced dimerization of this mutant using a cell-permeable bivalent compound led to its decreased internalization. Collectively, our studies demonstrate for the first time that the Dsg2 DUR mediates intermolecular interactions between the Dsg2 cytoplasmic domains, and that this function is linked to the inhibition of Dsg2 internalization. This discovery has important implications for normal tissue remodeling and homeostasis, in addition to the pathogenesis of diseases such as cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-32. doi:1538-7445.AM2012-LB-32