MULTIPLE KERATINS OF CULTURED HUMAN EPIDERMAL-CELLS ARE TRANSLATED FROM DIFFERENT MESSENGER-RNA MOLECULES

The keratins of human epidermis consist of several distinct proteins of different molecular weight that can be separated by gel electrophoresis in the presence of sodium dodecylsulfate. These proteins are very similar in structure, as determined by amino acid composition, polypeptide mapping and immunological reactivity. At least five such keratins are found in cultured human epidermal cells. We have examined the mode of synthesis of these keratins by isolating the poly(A)+ mRNA from the cultured cells and translating it in a reticulocyte system. All the keratins characteristic of the cultured cells were synthesized in vitro from the mRNA; they were identified by their molecular weight and by polypeptide mapping. No evidence was found for any precursor of substantially greater molecular weight. A study of the kinetics of synthesis showed that all the keratins were labeled within 2 min after the addition of 35S-methionine to a translation system preincubated with epidermal mRNA, and the relative intensities of labeling did not change upon further incubation. It was therefore improbable that one keratin could be the precursor of another. The mRNAs of the large keratins could be completely separated from those of the small keratins by gel electrophoresis under either native or denaturing conditions. Within the group of small mRNAs, each had a different mobility although resolution was incomplete. Upon translation, the mRNA fractions yielded different groups of keratins corresponding in molecular weight to their counterparts in the cells. Consequently, most if not all keratins of different size are translated from different messages. The approximate sizes of the mRNA molecules for different keratins were determined from their mobility under denaturing conditions. The size of the mRNA was not always proportional to the size of the encoded keratin, demonstrating the existence of noncoding segments of different length in the different mRNA molecules. © 1979.