Human transcortin synthesis by a cell-free translation of hepatic mRNA

To evaluate the site of synthesis and to characterize the translated transcortin, poly (A)-containing RNA (mRNA) from human liver was translated in a cell-free system derived from rabbit reticulocyte lysate. The in vitro synthesized product was identified as transcortin by immuno-precipitation with its specific antiserum. This translated transcortin could be displaced from the antibody by unlabeled purified transcortin obtained from plasma. Furthermore, when the translation mixture was applied to a cortisol-Sepharose column, the translated transcortin was bound to the matrix in a specific manner, indicating that this product binds to cortisol. The molecular weight of the translated transcortin was estimated to be 45,700 by its mobility in sodium dodecyl sulfate polyacrylamide gel electrophoresis, while that of plasma transcortin was 53,800. The difference in molecular weight between the translated transcortin and plasma transcortin was probably due to the presence of pre-sequence (signal peptide) in addition to the absence of carbohydrate moiety in the former. In conclusion, human liver mRNA directed the synthesis of transcortin, and the translated transcortin binds to cortisol in spite of the absence of carbohydrate moiety.