Antiserum to the stratum corneum basic protein (SCBP) of newborn rat epidermis was used to test for cross-reactive proteins in extracts of skin and in tissue sections. The antibody reacts strongly with epidermal extracts but very poor with dermal extracts. Buffer extracts of epidermis give a reaction of partial identity or identity with the antigen, SCBP. Urea extracts of isolated stratum corneum give a reaction of identity with SCBP. When the proteins of these extracts are separated by SDS polyacrylamide gel electrophoresis, the immunoreactive peak in the 4 M urea extract corresponds to the 49,000 MW SCBP. The immunoreactive peak in the 1 M potassium phosphate extract corresponds to a 52,000 MW protein. This protein is rapidly and transiently labeled after injection of 3H-histidine into newborn rats, in contrast to the SCBP which is labeled after a 5-hr lag. The 52,000 MW protein appears to be an immunologically related precursor of the SCBP. Immunoreactive proteins were localized in tissue sections by the indirect immunoperoxidase method. A strong positive reaction was seen in keratohyalin granules and in the stratum corneum. The reaction of keratohyalin granules corroborates the extraction of a cross-reactive protein by 1 M potassium phosphate, a method for extraction of keratohyalin granules from epidermis. These results are consistent with the hypothesis that a protein of 52,000 MW is present in keratohyalin and is converted to the SCBP (49,000 MW) concomitantly with the conconversion of a granular cell to a stratum corneum cell.

The fully differentiated anucleate cells of the stratum corneum of newborn rat epidermis contain a cationic protein called stratum corneum basic protein (SCBP). This protein has a molecular weight (49 000) and an amino acid composition similar to a protein extracted from the less differentiated cell layers of the epidermis. Pulse--chase experiments with radiolabeled histidine were undertaken to test the possiblity that SCBP is derived from a preexisting protein. A protein of 52 000 daltons is rapidly but transiently labeled in extracts of the less differentiated cell layers. As the amount of label in the 52 000-dalton protein decreases, an increase in radiolabel is observed in extracts of the fully differentiated cells. This label is found in SCBP, a protein of lower molecular weight (49 000) than that initially labeled. These proteins are immunologically related and both are resistant to cyanogen bromide cleavage. They differ in apparent molecular weight on sodium dodecyl sulfate--polyacrylamide gels and in their net charge. The results are consistent with the conversion of a precursor protein into SCBP.