Plakophilin 1 interferes with plakoglobin binding to desmoplakin, yet together with plakoglobin promotes clustering of desmosomal plaque complexes at cell-cell borders

Abstract

Desmosomes are adhesive junctions that link intermediate filament networks to sites of strong intercellular adhesion. These junctions play an important role in providing strength to tissues that experience mechanical stress such as heart and epidermis. The basic structural elements of desmosomes are similar to those of the better-characterized adherens junctions, which anchor actin-containing microfilaments to cadherins at the plasma membrane. This linkage of actin to classic cadherins is thought to occur through an indirect mechanism requiring the associated proteins, alpha- and beta-catenin. In the case of desmosomes, both linear and lateral interactions have been proposed as playing an important role in formation of the plaque and linkage to the cytoskeleton. However, the precise nature of these interactions and how they cooperate in desmosome assembly are poorly understood. Here we employ a reconstitution system to examine the assembly of macromolecular complexes from components found in desmosomes of the differentiated layers of complex tissues. We demonstrate the existence of a Triton-soluble complex of proteins containing full length desmoplakin (DP), the arm protein plakoglobin, and the cytoplasmic domain of the desmosomal cadherin, desmoglein 1 (Dsg1). In addition, full length DP, but not an N-terminal plakoglobin binding domain of DP, co-immunoprecipitated with the Dsg1 tail in the absence of plakoglobin in HT1080 cells. The relative roles of the arm proteins plakoglobin and plakophilin 1 (PKP1) were also investigated. Our results suggest that, in the Triton soluble pool, PKP1 interferes with binding of plakoglobin to full length DP when these proteins are co-expressed. Nevertheless, both plakoglobin and PKP1 are required for the formation of clustered structures containing DP and the Dsg1 tail that ultrastructurally appear similar to desmosomal plaques found in the epidermis. These findings suggest that more than one armadillo family member is required for normal assembly and clustering of the desmosomal plaque in the upper layers of the epidermis.