Structural proteins in perinatal rat epidermis: characterization of a high molecular weight prekeratin.

The major water-insoluble proteins of perinatal rat epidermis have been examined by gel electrophoretic techniques. Particular focus has been placed on that family of epidermal structural proteins called keratins which are characterized by mol wt between 40 and 70 kD. Analysis of these proteins by 2-dimensional PAGE revealed the largest member of this family (Mr = 63 kD) to consist of a series of isoelectric variants with isoelectric points ranging between 7.3 and 5.9. Antibodies raised in rabbits against this protein were specific by immunoblot analysis and exhibited no cross-reactivity with keratins isolated from human foreskin epidermis under the same extraction conditions. Ontogenetic examination by Western blot was performed on extracts of whole fetal rat skin from d 17 to d 19 of gestation. Expression of the protein was seen only after the 18th gestational d. Posttranslational modification of neonatal rat keratins by phosphorylation was examined under in vitro conditions at two different ambient temperatures (23 and 37 degrees C). Overall phosphorylation was markedly increased at the higher temperature. A similar qualitative pattern of keratin phosphorylation was seen after in vivo labeling at nest temperature (35 degrees C). In both the in vitro and in vivo experiments, the major radiolabeled moiety was the 63 kD epidermal protein. In summary, insoluble proteins between 40 and 70 kD have been examined in perinatal rat epidermis. The tissue localization, solubility, phosphorylation status, ontogenetic appearance, and mol wt of the 63 kD protein are consistent with the identification of an epidermal prekeratin. We hypothesize that this protein is an important molecular precursor of stratum corneum formation in the perinatal rat.