BIOSYNTHESIS OF BOVINE PLASMA-PROTEINS IN A CELL-FREE SYSTEM - AMINO-TERMINAL SEQUENCE OF PREPROALBUMIN

Preparative amounts of poly(A)-containing RNA were isolated from bovine liver. This RNA stimulated protein synthesis in an mRNA-dependent rabbit reticulocyte lysate system employing various radioactively labeled amino acids. After translation, several plasma proteins were identified by immunoprecipitation, including albumin, fibrinogen, prothrombin and antithrombin III. Automatic Edman degradation of immunoprecipitated albumin showed that this protein was synthesized as a precursor (preproalbumin) containing an N-terminal extension of 18 amino acid residues. The N-terminal sequence was as follows: .**GRAPHIC**. This extension contains many hydrophobic amino acid residues and corresponds to the signal region found in other secreted proteins. Residues 19-24 correspond to the pro region of bovine proalbumin, and Asp-25 to the N terminus of albumin found in plasma. The N-terminal methionine of preproalbumin is provided by the initiator .**GRAPHIC**. species. Comparison of the signal sequence of bovine preproalbumin with that from rat showed considerable homology between the 2 (11 of 18 residues in corresponding positions were identical), but comparison of the signal sequence with that of other bovine proteins, such as preproparathyroid hormone, showed no homology.