Synthesis of secretory proteins in developing mouse yolk sac

Synthesis of secretory proteins in the developing mouse visceral yolk sac was studied. Newly synthesized proteins were labeled with ³⁵S]methionine and characterized by two-dimensional gel electrophoresis. A large increase in the relative rate of synthesis of a small number of proteins occurred between Days 9.5 and 15.5 of development. These proteins were the predominant proteins synthesized and secreted by the yolk sac throughout this period of gestation. Two of these proteins were identified as α-fetoprotein and transferrin by specific immunoprecipitation. α-Fetoprotein synthesis increased from about 3% of the total protein synthesis at Day 9.5 to about 26% at Day 15.5 after which it declined slightly. The relative rate of transferrin synthesis had a similar developmental pattern, reaching the highest level (5%) at Day 15.5, but declined more rapidly than α-fetoprotein synthesis. Quantitatively, these two proteins represented about 60% of the total secreted protein. Gestational changes in the content of α-fetoprotein messenger RNA were determined by hybridization analysis using α-fetoprotein complementary DNA probe. The percentage of α-fetoprotein messenger RNA in total yolk sac RNA increased about ninefold from Day 9.5 to Day 14.5. This increase correlated well with the increase in the relative rate of α-fetoprotein synthesis during the identical period. This study suggests that after Day 9.5 the yolk sac is completing a differentiation process which is characterized by the preferential expression of a small group of secretory protein genes.