Program of early development in the mammal: Synthesis of mitochondrial proteins during oogenesis and early embryogenesis in the mouse

During oogenesis and early embryogenesis in the mouse, mitochondria of growing oocytes and cleaving embryos undergo extensive morphological alterations (e.g., L. Pikó and D.G. Chase, 1973, J. Cell Biol. 58, 357; P.M. Wassarman and W. J. Josefowicz, 1978, J. Morphol. 156, 209). The experiments described here were carried out to determine whether or not proteins encoded by the mitochondrial genome are synthesized during these stages of development. Proteins synthesized by growing oocytes, fully grown oocytes, and eight-cell compacted embryos cultured in the presence or absence of emetine, an inhibitor of cytoplasmic protein synthesis, were examined by one- and two-dimensional gel electrophoresis. Electrophoretic patterns of [³⁵S]methionine-labeled proteins in these cells were compared with each other, as well as with patterns of proteins synthesized in mouse follicle cells cultured in the presence or absence of emetine. In the presence of emetine, all of these cell types synthesize a very similar array of proteins and synthesis of these proteins is arrested by chloramphenicol, an inhibitor of mitochondrial protein synthesis. The molecular weights of the proteins synthesized in the presence of emetine are very similar to those of mitochondrial proteins synthesized in a variety of eukaryotic cells. Synthesis of mitochondrial proteins accounts for less than 2% of total protein synthesis in mouse oocytes, embryos, and follicle cells, such that they represent minor components in cells radiolabeled in the absence of emetine. Consequently, electrophoretic patterns of proteins radiolabeled in cells cultured in the presence of chloramphenicol are indistinguishable from those of cells cultured in the absence of the drug. On the other hand, electrophoretic patterns of preparations enriched in oocyte or follicle cell mitochondria show an enrichment of mitochondrial-encoded proteins. These results demonstrate that mitochondrial proteins are synthesized during oocyte growth and early embryogenesis in the mouse, despite the marked morphological alterations undergone by mitochondria during these same stages of development. In addition, these and other (L. Pikó and D. G. Chase, 1973, J. Cell Biol. 58, 357) observations suggest that the mitochondrial transformations are under nucleocytoplasmic, rather than mitochondrial, control.