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

Phosphatidate phosphatases (PAH) play a central role in lipid metabolism and intracellular signaling. Herein, we report the presence of a low-molecular-weight PAH homolog in the single-celled ciliate Tetrahymena thermophila. In vitro phosphatase assay showed that TtPAH2 belongs to the magnesium-dependent phosphatidate phosphatase (PAP1) family. Loss of function of TtPAH2 did not affect the growth of Tetrahymena. Unlike other known PAH homologs, TtPAH2 did not regulate lipid droplet number and ER morphology. TtPAH2 did not rescue growth and ER/nuclear membrane defects of the pah1Δ yeast cells, suggesting that the phosphatidate phosphatase activity of the protein is not sufficient to perform these cellular functions. Surprisingly, TtPAH2 complemented the respiratory defect in the pah1Δ yeast cells indicating a specific role of TtPAH2 in respiration. Overall, our results indicate that TtPAH2 possesses the minimal function of PAH protein family in respiration. We suggest that the amino acid sequences absent from TtPAH2 but present in all other known PAH homologs are critical for lipid homeostasis and membrane biogenesis.