THE SINGLE TRANSLATION PRODUCT OF THE FUM1 GENE (FUMARASE) IS PROCESSED IN MITOCHONDRIA BEFORE BEING DISTRIBUTED BETWEEN THE CYTOSOL AND MITOCHONDRIA IN SACCHAROMYCES-CEREVISIAE

The yeast mitochondrial and cytosolic isoenzymes of fumarase, which ar e encoded by a single nuclear gene (FUM1), follow a unique mechanism o f protein subcellular localization and distribution. Translation of al l FUM1 messages initiates only from the 5'-proximal AUG codon and resu lts in a single translation product that contains the targeting sequen ce located within the first 32 amino acids of the precursor. All fumar ase molecules synthesized in the cell are processed by the mitochondri al matrix signal peptidase; nevertheless, most of the enzyme (80 to 90 %) ends up in the cytosol. The translocation and processing of fumaras e are cotranslational. We suggest that in Saccharomyces cerevisiae, th e single type of initial translation product of the FUM1 gene is first partially translocated, and then a subset of these molecules continue s to be fully translocated into the organelle, whereas the rest are fo lded into an import-incompetent state and are released by the retrogra de movement of fumarase into the cytosol.