Binding of keratin intermediate filaments (K10) to the cornified envelope in mouse epidermis: implications for barrier function.

The cornified envelope, a structure unique to keratinocytes, is a hallmark of their terminal differentiation and plays an important role in epidermal barrier function. Cornified envelope is formed through the action of a membrane-associated transglutaminase, which covalently cross-links protein precursors into a highly insoluble network at the inner leaflet of the plasma membrane in granular keratinocytes and stratum corneum. Initial studies, using dansylcadaverine for enzyme-directed labeling of acyl-acceptor transglutaminase substrates in mouse epidermal homogenates identified a prominent 60-kDa substrate. Specific antibodies raised to this protein stained the cytoplasm of suprabasal cells of stratified squamous epithelia, whereas simple epithelia and nonepithelial tissues showed no staining. Immunoscreening of a cDNA expression library from adult mouse skin identified 18 positive clones. DNA sequencing of the largest clone (which hybridized to a keratinocyte-specific transcript of 2.0 kb) showed greater than 99.5% homology with mouse keratin 10. Immunoelectron microscopy using anti-S60 and another antibody to keratin 10 showed specific binding to cornified envelope associated filamentous structures. Proteolytic fragments of purified cornified envelope from mouse epidermis showed reactivity to anti-S60. These data show that mouse keratin 10 is tightly bound to cornified envelope and may be a cross-linked substrate. The tight binding of keratin filaments and CE suggests a mechanism by which they might interact to enhance the structural integrity of the stratum corneum.