Histidine-rich proteins as molecular markers of epidermal differentiation.

Cells of the basal, spinous, and granular layers of the mammalian epidermis represent morphological stages in the differentiation of keratinocytes to form the outer cornified layer of the skin. In the newborn rat, concomitant with the transition of granular into cornified cells, there is a conversion of histidine-rich protein I (HRP,), localized in keratinocytes of the epidermal granular layer, into HRPII. This transition from HRP, to HRP” provides a biochemical marker for this particular differentiative event. HRPl and HRPll have similar amino acid compositions which are unusually high in polar amino acids (greater than 80 mol %) and histidine (approximately 7 mol %). As estimated from gel filtration studies in the presence of sodium dodecyl sulfate, HRP, exhibits a broad range of molecular weights from approximately 60,000 to greater than lo6 whereas HRPll has an apparent molecular weight between 60,000 and 80,000. Peptide mapping of a tryptic digest of a mixture of HRPl labeled with [14C]histidine and HRPrr labeled with [3H]histidine shows that of 18 peptides which are significantly labeled with 3H and a similar number labeled with 14C, 11 of the 3H-peptides chromatograph coincidentally with 14C-peptides. HRPl accounts for about 40% of the total nondialyzable 3H in the epidermis after exposure in uiuo for 1 h to C3H]histidine and about 30% after a similar exposure to an equally labeled mixture of the 20 common 3Hamino-acids. HRPl constitutes only about 4% of the epidermal protein which is soluble in 8 M urea, 0.2 M Tris, pH 8.5, whereas HRP,, is a major component and represents 35% of this extract. HRPn is present primarily in the cells of the lower cornified layers as demonstrated by autoradiography of histologically “pure” stratum corneum and isolation of the substance from such tissue preparations. HRP, is localized in the cells of the granular layers. Available data support the hypothesis that the product of translation, HRPo, is polymerized rapidly in the granular cells to a large polymer of heterogeneous size,