Proteolytic modification of acidic and basic keratins during terminal differentiation of mouse and human epidermis

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Abstract
Keratins from the living cell layers of human and neonatal mouse epidermis (prekeratins) were compared to those from the stratum corneum (SC keratins). Human and mouse epidermis contained 4 prekeratins, 2 of each keratin subfamily: type II basic (pI 6.5-8.5; human 68 kDa [kilodalton], 60.5 kDa and mouse 67 kDa, 60 kDa) and type I acidic (pI 4.7-5.7; human 57 kDa, 51 kDa and mouse 58 kDa, 53 kDa). While all 4 were present in equal amounts in adult human epidermis, 2 (67 kDa basic, 58 kDa acidic) were more prominent in neonatal mouse epidermis. Preliminary results with cell fractions (basal, spinous and granular) indicated that quantitative differences were a function of morphology, basal cells containing the smaller membrane of each subfamily and granular cells the larger. Mouse stratum corneum extracts contained 4 keratins (3 in human): type II neutral-acidic (pI 5.7-6.7; human 65 kDa and mouse 64 ka, 62 kDa) and type I acidic (pI 4.9-5.4; human 57.5 kDa, 55 kDa and mouse 58.5 kDa, 57.5 kDa). In both species, 1-dimensional and 2-dimensional peptide mapping (with V8 protease and trypsin, respectively) indicated that while all 4 prekeratins were distinct gene products, similarities existed in the type II basic and the type I acidic keratin subfamilies. A strong homology also existed between type II SC keratins and the larger basic (type II) prekeratin (human 68 kDa and mouse 67 kDa), and between type I SC keratins and the larger acidic (type I) prekeratin (human 57 kDa and mouse 58 kDa). A precursor-product relationship is indicated within each keratin subfamily, between SC keratins and the prekeratins abundant in the adjacent granular layer. This differentiation-related keratin processing was similar in mouse and human epidermis, and may represent a widespread phenomenon amongst keratinizing epithelia.