Mutations Altering the Mitochondrial-Cytoplasmic Distribution of Mod5p Implicate the Actin Cytoskeleton and mRNA 3′ Ends and/or Protein Synthesis in Mitochondrial Delivery

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Abstract
The Saccharomyces cerevisiae MOD5 gene encodes proteins that function in three subcellular locations: mitochondria, the cytoplasm, and nuclei (M. Boguta, L.A. Hunter, W.-C. Shen, E. C. Gillman, N. C. Martin, and A. K. Hopper, Mol. Cell. Biol. 14:2298-2306, 1994; E. C. Gillman, L. B. Slusher, N. C. Martin, and A. K. Hopper, Mol. Cell. Biol. 11:2382-2390, 1991). A mutant allele of MOD5 encoding a protein (Mod5p-I,KR6) located predominantly in mitochondria was constructed. Mutants defective in delivering Mod5p-I,KR6 to mitochondria were sought by selecting cells with increased cytosolic activity of this protein. Twenty-five mutants defining four complementation groups, mdp1, mdp2, mdp3, and mdp4, were found. They are unable to respire at 34 degrees C or to grow on glucose medium at 38 degrees C. Cell fractionation studies showed that mdp1, mdp2, and mdp3 mutants have an altered mitochondrial-cytoplasmic distribution of Mod5p. mdp2 can be suppressed by ACT1, the actin-encoding gene. The actin cytoskeleton organization is also aberrant in mdp2 cells. MDP2 is the same as VRP1 (S. F. H. Donnelly, M. J. Picklington, D. Pallotta, and E. Orr, Mol. Microbiol. 10:585-596, 1993). MDP3 is identical to PAN1, which encodes a protein that interacts with mRNA 3' ends and affects initiation of protein synthesis (A. B. Sachs and J. A. Deardoff, Cell 70:961-973, 1992). These results implicate the actin cytoskeleton and mRNA 3' ends and/or protein synthesis as being as important for protein distribution in S. cerevisiae as they are for distribution of cytosolic proteins in higher eukaryotes. This provides the potential to apply genetic and molecular approaches to study gene products and mechanisms involved in this type of protein distribution. The selection strategy also offers a new approach for identifying gene products involved in the distribution of proteins to their subscellular destinations.