Program of early development in the mammal: Post-transcriptional control of a class of proteins synthesized by mouse oocytes and early embryos

Abstract

Following fertilization of mouse eggs there is a singular change in the pattern of newly synthesized proteins, as analyzed by high-resolution two-dimensional gel electrophoresis. The synthesis of six polypeptides, designated here as “fertilization proteins” FP1-FP6, is greatly enhanced in one-cell embryos as compared to ovulated eggs. Their synthesis continues for about 36 hr, during which the first cleavage division takes place, and is virtually terminated by the eight-cell stage of development. These six proteins all have apparent molecular weights of approximately 35,000 daltons and isoelectric points ranging from about 6.1 (FP1) to 6.9 (FP6). Peptide mapping of [ 35S]methionine-labeled FP1-FP4 on SDS-polyacrylamide gels revealed very similar electrophoretic patterns of radiolabeled peptides, suggesting that the four polypeptides are structurally related to one another. Although FP1-FP4 are synthesized by mouse oocytes, they represent minor products, with only about 0.2% of the [ 35S]methionine incorporated into oocyte proteins present in FP1-FP4 in growing and fully grown oocytes. This is to be compared with the synthesis of lactate dehydrogenase (LDH) and tubulin (α and β) which are major products, representing as much as 1.8 and 2.0%, respectively, of total protein synthesis in growing and fully grown oocytes. On the other hand, in an mRNA-dependent, cell-free translation system (rabbit reticulocyte lysate) to which RNA prepared from mouse oocytes is added, the incorporation of [ 35S]methionine into FP1, FP2, FP3, or FP4 is comparable to incorporation into either LDH or tubulin. Along with LDH, tubulin, and actin, FP1-FP4 are the major cell-free translation products observed on fluorograms of two-dimensional gels; such is the case using RNA prepared from oocytes at all stages of growth (20 to 85 μm in diameter). Comparisons of [ 35S]methionine incorporation into FP1-FP4, LDH, and tubulin in cultured oocytes and in a cell-free translation system indicate that the synthesis of FP1-FP4 in mouse oocytes is very low relative to levels of mRNA coding for these proteins that are present throughout oocyte growth. The results suggest that expression of FP1-FP4 during oogenesis is under post-transcriptional control, and that following fertilization of mouse eggs there is a marked stimulation of translation of mRNA coding for FP1-FP4. It is possible that the mRNA may have been transcribed from days to weeks earlier during oocyte growth.