Articles written in Journal of Biosciences

    • Small phosphatidate phosphatase (TtPAH2) of Tetrahymena complements respiratory function and not membrane biogenesis function of yeast PAH1


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      Phosphatidate phosphatases (PAH) play a central role in lipid metabolism and intracellular signaling. Herein, we report thepresence of a low-molecular-weight PAH homolog in the single-celled ciliate Tetrahymena thermophila. In vitro phosphataseassay showed that TtPAH2 belongs to the magnesium-dependent phosphatidate phosphatase (PAP1) family. Loss offunction of TtPAH2 did not affect the growth of Tetrahymena. Unlike other known PAH homologs, TtPAH2 did not regulatelipid droplet number and ER morphology. TtPAH2 did not rescue growth and ER/nuclear membrane defects of the pah1Dyeast cells, suggesting that the phosphatidate phosphatase activity of the protein is not sufficient to perform these cellularfunctions. Surprisingly, TtPAH2 complemented the respiratory defect in the pah1D yeast cells indicating a specific role ofTtPAH2 in respiration. Overall, our results indicate that TtPAH2 possesses the minimal function of PAH protein family inrespiration. We suggest that the amino acid sequences absent from TtPAH2 but present in all other known PAH homologsare critical for lipid homeostasis and membrane biogenesis.

    • A putative NEM1 homologue regulates lipid droplet biogenesis via PAH1 in Tetrahymena thermophila


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      Nuclear envelope morphology protein 1 (NEM1) along with a phosphatidate phosphatase (PAH1) regulates lipid homeostasisand membrane biogenesis in yeast and mammals. We investigated four putative NEM1 homologues (TtNEM1A,TtNEM1B, TtNEM1C and TtNEM1D) in the Tetrahymena thermophila genome. Disruption of TtNEM1B, TtNEM1C orTtNEM1D did not compromise normal cell growth. In contrast, we were unable to generate knockout strain of TtNEM1Aunder the same conditions, indicating that TtNEM1A is essential for Tetrahymena growth. Interestingly, loss of TtNEM1Bbut not TtNEM1C or TtNEM1D caused a reduction in lipid droplet number. Similar to yeast and mammals, TtNem1B ofTetrahymena exerts its function via Pah1, since we found that PAH1 overexpression rescued loss of Nem1 function.However, unlike NEM1 in other organisms, TtNEM1B does not regulate ER/nuclear morphology. Similarly, neitherTtNEM1C nor TtNEM1D is required to maintain normal ER morphology. While Tetrahymena PAH1 was shown tofunctionally replace yeast PAH1 earlier, we observed that Tetrahymena NEM1 homologues did not functionally replaceyeast NEM1. Overall, our results suggest the presence of a conserved cascade for regulation of lipid homeostasis andmembrane biogenesis in Tetrahymena. Our results also suggest a Nem1-independent function of Pah1 in the regulation ofER morphology in Tetrahymena.

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