A Biochemical Marker in Epidermal Differentiation

Unusual protein containing a high level of histidine and eight other amino acids (i.e. glycine, alanine, serine, threonine, arginine, glutamic acid, aspartic acid and tyrosine) is localized in the epidermal granular cell layer of the newborn rat. I t appears that the formation of "histidine-protein" is a step in the sequence of epidermal differentiation terminating in keratinization. This paper reports the biosynthesis of "histidine-protein" by granular cells, in vitro. The protein, having a molecular weight of about 30 ooo, is obtained by extraction of homogenized epidermis in 8 M urea, dialysis of the extracted protein, lyophilization of the non-dialyzable material, extraction of the dried residue at 24 ° with o.I M HC104, precipitation at pH 4.5 and purification on Sephadex G-Ioo. Synthesis of "histidine-protein" appears to occur in the "large" microsomal fraction and to involve at least two steps. A "peptide precursor", formed initially in a reaction sensitive to puromycin, is incorporated into "histidine-protein" by a mechanism not blocked by this inhibitor.