Translational products of mRNAs coding for non-epidermal cytokeratins

Total RNA and poly(A)+ RNA were isolated from tissues and cultured cells of various mammalian species (bovine muzzle epidermis and bladder urothelium; rat hepatoma cells; human cell lines HeLa, MCF-7 and A-431) and examined by translation in vitro using the reticulocyte lysate system. Polypeptides were separated and identified by two-dimensional electrophoresis and cytokeratins were selectively enriched from the translation assays by co-polymerization with added heterologous cytokeratins. In all three species, non-epidermal cytokeratins A, D and mol. wt. 40,000 (corresponding to numbers 8, 18 and 19 of the human cytokeratin catalog of Moll et al., 1982) were identified as translation products capable of co-polymerization with epidermal keratins. Several other basic and other acidic cytokeratins were also identified as translational products. In addition, two unidentified polypeptides (mol. wt. 52,000 and 43,000) which were minor polypeptides in cytoskeletons and translation assays were found to be specifically enriched in co-polymers with bovine epidermal keratins. The results indicate that many, perhaps all, non-epidermal cytokeratins characteristic of simple epithelia are genuine products of translation and that their diversity is not due to post-translational modification or processing. These findings, taken together with observations of in vitro translation of epidermal mRNAs, suggest that the diversity of cell type-specific expression of the different members of the cytokeratin polypeptide family is largely due to the cell type-specific synthesis of diverse mRNAs.