In vitro synthesis of murine pre-alpha-fetoprotein

Murine alpha-fetoprotein was synthesized in a wheat germ cell-free system in the presence of radioactive amino acids under the direction of alpha-fetoprotein messenger RNA isolated from mouse yolk sacs. The radiolabeled alpha-fetoprotein was isolated by immunoabsorption, and the amino acid residues at the NH2 terminus were determined by radioactive sequencing techniques. In a comparison to the NH2-terminal sequence of circulating alpha-fetoprotein, the in vitro-synthesized alpha-fetoprotein was found to contain an extra peptide 20 amino acids long linked at the NH2 terminus, the sequence of which is: (formula: see text). The molecular size, the hydrophobic nature, and the other properties of the peptide are consistent with the "leader" or "signal" piece found in the precursors of many other secretory proteins. This suggests that alpha-fetoprotein, the synthesis of which is limited primarily to fetal development, is produced in the form of the precursor as are secretory proteins in the adult tissues.