"HERE AND ALSO THERE": MECHANISMS, FUNCTIONS AND EVOLUTION OF DUAL LOCALIZED PROTEINS

In eukaryotic cells, identical proteins can be located in more than a single subcellular compartment, a phenomenon termed dual targeting. We have shown that dual targeting is a highly abundant phenomenon in eukaryotes and in fact we estimate that in yeast a third of the mitochondrial proteome is dual targeted. To examine the evolutionary conservation of dual targeted proteins we employed codon usage bias, propensity for gene loss, phylogenetic relationships, conservation analysis at the DNA level, and gene expression. Our findings indicate that, dual-targeted proteins are significantly more conserved than their exclusively targeted counterparts. Our hypothesis is that this conservation is due to separate selective pressures administered by the different compartments to maintain the dual functions.

Fumarase (FH, fumarate hydratase) and aconitase are examples of Krebs/TCA cycle enzymes that are distributed between two compartments of the eukaryotic cell. Interestingly, fumarase was previously shown to underlie a tumor susceptibility syndrome, Hereditary Leiomyomatosis and Renal Cell Cancer (HLRCC). Aconitase is also associated with a human genetic disease, encephalopathy in infancy. Both fumarase and aconitase distribute between the mitochondria, and the cytosol by different mechanisms in different organisms. The cytosolic version of fumarase is involved in the DNA damage response (DDR), while aconitase is a component of the glyoxylate shunt in yeast and plants while a version of this latter enzyme is involved in the regulation of iron metabolism in mammalian cells. We now show that fumarase of the prokaryote Bacillus subtilis is induced upon DNA damage, co-localized with the bacterial DNA and is required for the DDR. Our studies provide an evolutionary model in which the dual function of the ancient prokaryotic enzymes, was the driving force for its targeting into different cellular compartments in eukaryotes.