The mitochondrial and cytosolic forms of avian phosphoenolpyruvate carboxykinase (GTP) are encoded by different messenger RNAs.

Previous work from our laboratory (Watford, M., Hod, Y., Chiao, Y. B., Utter M. F., and Hanson R. W. (1981) J. Biol. Chem. 256, 10023-10027) indicated that in the chicken, hepatic phosphoenolpyruvate carboxykinase is in the mitochondria, whereas kidney contains both a mitochondrial and cytosolic form of the enzyme. In the present study the two forms of phosphoenolpyruvate carboxykinase were purified and shown to be distinct proteins which differ in size, charge, and immunochemical properties. Using a cell-free protein synthesis system, we demonstrate that the cytosolic isozyme is encoded only by kidney mRNA and that its translation form has similar properties to that of the mature protein. On the other hand, the mitochondrial enzyme is encoded by liver and kidney mRNA and synthesized as a protein about 2000 daltons larger than the mature form. The putative precursor of the mitochondrial phosphoenolpyruvate carboxykinase is processed to a mature size by isolated, respiring mitochondria. We further show that the mRNA species for the cytosolic and mitochondrial forms are separable and are about 3 and 4 kilobases, respectively. It is concluded that the two forms of phosphoenolpyruvate carboxykinase of the chicken are encoded by distinct mRNA species.