MAKING A CONNECTION - DIRECT BINDING BETWEEN KERATIN INTERMEDIATE FILAMENTS AND DESMOSOMAL PROTEINS

In epidermal cells, keratin intermediate filaments connect with desmos omes to form extensive cadherin-mediated cytoskeletal architectures. D esmoplakin (DPI), a desmosomal component lacking a transmembrane domai n, has been implicated in this interaction, although most studies have been conducted with cells that contain few or no desmosomes, and effo rts to demonstrate direct interactions between desmoplakin and interme diate filaments have not been successful. In this report, we explore t he biochemical nature of the connections between keratin filaments and desmosomes in epidermal keratinocytes. We show that the carboxy termi nal ''tail'' of DPI associates directly with the amino terminal ''head '' of type II epidermal keratins, including K1, K2, K5, and K6. We hav e engineered and purified recombinant K5 head and DPI tail, and we dem onstrate direct interaction in vitro by solution-binding assays and by ligand blot assays. This marked association is not seen with simple e pithelial type II keratins, vimentin, or with type I keratins, providi ng a possible explanation for the greater stability of the epidermal k eratin filament architecture over that of other cell types. We have id entified an 18-amino acid residue stretch in the K5 head that is conse rved only among type II epidermal keratins and that appears to play so me role in DPI tail binding. This finding might have important implica tions for understanding a recent point mutation found within this bind ing site in a family with a blistering skin disorder.