Many faces of periplakin: domain-specific antibodies detect the protein throughout the epidermis, explaining the multiple protein-protein interactions.

Periplakin was first identified as a precursor of the cornified cell envelope, residing in desmosomes and in interdesmosomal plasma membrane of differentiated keratinocytes in epidermis. However, most antibodies used so far have been raised against the C-terminal peptide of periplakin, the region which is heavily involved in protein-protein interactions. In order to avoid the lack of signal due to the epitope masking, other regions of periplakin were selected as antigens. Indeed, periplakin is present throughout the epidermis, but while the head domain is accessible for the antibody recognition throughout the epidermis, the rod domain is readily accessible only in the basal cell layer and needs to be unmasked by detergent treatment in the upper layers of epidermis. The tail domain, which is known to be heavily involved in the protein-protein interactions, specifically binding to the intermediate filament proteins, was masked in the basal cell layer, but was readily detectable in the outer layers of epidermis, where periplakin assembles to the cornified cell envelope. The in vitro assay revealed that periplakin tail serves as a substrate for the tissue-type transglutaminase, TGase 2. The cross-linking may potentially render the tail unaccessible for antibody recognition. A tailless periplakin, unable to function as a cytolinker, results from the proteolytic processing of periplakin by caspase 6, the caspase executing apoptosis through the processing of intermediate filament proteins. It is concluded that multiple localization and protein-protein interactions indicate a functional role for periplakin and its domains throughout the epithelial differentiation.