Compartment-specific isoforms of TPI and GAPDH are imported into diatom mitochondria as a fusion protein: Evidence in favor of a mitochondrial originof the eukaryotic glycolytic pathway

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and triosephosphate isomer ase (TPI) are essential to glycolysis, the major route of carbohydrate brea kdown in eukaryotes. In animals and other heterotrophic eukaryotes, both en zymes are localized in the cytosol; in photosynthetic eukaryotes, GAPDH and TPI exist as isoenzymes that function in the glycolytic pathway of the cyt osol and in the Calvin cycle of chloroplasts. Here, we show that diatoms-ph otosynthetic protists that acquired their plastids through secondary symbio tic engulfment of a eukaryotic rhodophyte-possess an additional isoenzyme e ach of both GAPDH and TPI. Surprisingly, these new forms are expressed as a n TPI-GAPDH fusion protein which is imported into mitochondria prior to its assembly into a tetrameric bifunctional enzyme complex. Homologs of this t ranslational fusion are shown to be conserved and expressed also in nonphot osynthetic, heterokont-flagellated oomycetes. Phylogenetic analyses show th at mitochondrial GAPDH and its N-terminal TPI fusion branch deeply within t heir respective eukaryotic protein phylogenies, suggesting that diatom mito chondria may have retained an ancestral state of glycolytic compartmentatio n that existed at the onset of mitochondrial symbiosis. These findings stro ngly support the view that nuclear genes for enzymes of glycolysis in eukar yotes were acquired from mitochondrial genomes and provide new insights int o the evolutionary history (host-symbiont relationships) of diatoms and oth er heterokont-flagellated protists.