A putative homologue regulates lipid droplet biogenesis via PAH1 in .

Nuclear envelope morphology protein 1 along with a phosphatidate phosphatase regulates lipid homeostasis and membrane biogenesis in yeast and mammals. We investigated four putative homologues and ) in the genome. Disruption of or did not compromise normal cell growth. In contrast, we were unable to generate knockout strain of under the same conditions, indicating that is essential for growth. Interestingly, loss of TtNEM1B but not TtNEM1C or TtNEM1D caused a reduction in lipid droplet number. Similar to yeast and mammals, TtNem1B of Tetrahymena exerts its function via Pah1, since we found that PAH1 overexpression rescued loss of Nem1 function. However, unlike NEM1 in other organisms, does not regulate ER/nuclear morphology. Similarly, neither TtNEM1C nor TtNEM1D is required to maintain normal ER morphology. While Tetrahymena PAH1 was shown to functionally replace yeast earlier, we observed that homologues did not functionally replace yeast NEM1. Overall, our results suggest the presence of a conserved cascade for regulation of lipid homeostasis and membrane biogenesis in . Our results also suggest a Nem1-independent function of Pah1 in the regulation of ER morphology in .