Abstract
Plakophilins (pkp‐1, ‐2, and ‐3) comprise a family of armadillo‐repeat containing proteins that are found in the desmosomal plaque and in the nucleus. Plakophilin‐1 is most highly expressed in the suprabasal layers of the epidermis and loss of plakophilin‐1 expression results in skin fragility‐ectodermal dysplasia syndrome, which is characterized by a reduction in the number and size of desmosomes in the epithelia of affected individuals. To investigate the role of plakophilin‐1 during desmosome formation, we fused plakophilin‐1 to the hormone‐binding domain of the estrogen receptor to create a fusion protein (plakophilin‐1/ER) that can be activated in cell culture by the addition of 4‐hydroxytamoxifen. When plakophilin‐1/ER was expressed in A431 cells it was incorporated into endogenous desmosomes and did not disrupt desmosome formation. A derivative of A431 cells (A431D) do not form desmosomes, even though they express all the components believed to be necessary for desmosome assembly. Expression and activation of plakophilin‐1/ER in A431D cells resulted in punctate desmoplakin staining on the cell surface. Co‐expression of a classical cadherin (N‐cadherin) and plakophilin‐1/ER in A431D cells resulted in punctate desmoplakin staining at cell–cell borders. These data suggest that plakophilin‐1 can induce assembly of desmosomal components in A431D cells in the absence of a classical cadherin; however a classical cadherin (N‐cadherin) is required to direct assembly of desmosomes between adjacent cells. The activatable plakophilin‐1/ER system provides a unique culture system to study the assembly of the desmosomal plaque in culture. © 2005 Wiley‐Liss, Inc.