Precursor of rat liver alpha 2u-globulin: partial amino acid sequence determination of its signal peptide.

The biosynthesis of alpha 2 u-globulin, the major protein in rat urine, was studied in a cell-free system programmed with messenger RNA isolated from rat liver. Analysis of the primary translation product by SDS-polyacrylamide gel electrophoresis showed a molecular weight of approximatley 22,000 daltons as compared to 20,000 daltons for the mature form. When translation was carried out in the presence of dog pancreas microsomes, alpha 2u-globulin was cotranslationally processed and sequestered in the microsomal lumen. Automated sequencing to mature alpha 2u-globulin and the labeled precursor disclosed the presence of 19 additional amino acids at the amino-terminal end of the primary translation product of the urinary alpha 2u-globulin. The partial amino acid sequence of this extension was determined.