Identification of pre-osteonectin produced by cell-free translation of fetal porcine calvarial mRNA.

The cell-free biosynthesis of the bone protein osteonectin was studied using mRNA from fetal porcine calvariae. Total RNA was extracted from the calvariae with guanidinium thiocyanate and was partially purified by precipitation with acid/ethanol. Translations were performed using the reticulocyte lysate system and were optimized with respect to mRNA concentration and K+ (70 mM) and Mg2+ (0.6 mM) concentration. Cell-free synthesized osteonectin, radiolabeled with [35S]methionine, was specifically immunoprecipitated with rabbit antiserum to porcine osteonectin and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. When analyzed under reduced conditions, the translated protein migrated with an Mr 45,000 compared to an Mr 39,000 for cell-synthesized osteonectin. When translated in the presence of microsomal membranes, the immunoprecipitated osteonectin co-migrated with the cell-synthesized osteonectin, indicating that a signal sequence of about 45-50 amino acids (Mr 6,000) had been removed. Under nonreduced conditions the pre-osteonectin co-migrated with osteonectin (Mr 39,000) on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting that a highly folded structure is retained by disulfide bridges under denaturing conditions. The relationship between the immunoprecipitated pre-osteonectin from the cell-free translations and both the cell-synthesized and tissue-extracted osteonectin was confirmed by one-dimensional peptide mapping of Staphylococcus aureus V-8 protease digestions. The results indicate that porcine osteonectin is synthesized on polysomes in a pre-osteonectin form which is translocated vectorially into microsomal vesicles and cotranslationally processed by the removal of a signal peptide.