Articles written in Journal of Biosciences

    • Regulation of dynamin family proteins by post-translational modifications


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      Dynamin superfamily proteins comprising classical dynamins and related proteins are membrane remodelling agentsinvolved in several biological processes such as endocytosis, maintenance of organelle morphology and viralresistance. These large GTPases couple GTP hydrolysis with membrane alterations such as fission, fusion ortubulation by undergoing repeated cycles of self-assembly/disassembly. The functions of these proteins are regulatedby various post-translational modifications that affect their GTPase activity, multimerization or membrane association.Recently, several reports have demonstrated variety of such modifications providing a better understanding of themechanisms by which dynamin proteins influence cellular responses to physiological and environmental cues. In thisreview, we discuss major post-translational modifications along with their roles in the mechanism of dynaminfunctions and implications in various cellular processes.

    • 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.

    • Tetrahymena dynamin-related protein 6 self-assembles independent of membrane association


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      Self-assembly on target membranes is one of the important properties of all dynamin family proteins. Drp6, a dynaminrelatedprotein in Tetrahymena, controls nuclear remodelling and undergoes cycles of assembly/disassembly on the nuclearenvelope. To elucidate the mechanism of Drp6 function, we have characterized its biochemical and biophysical propertiesusing size exclusion chromatography, chemical cross-linking and electron microscopy. The results demonstrate that Drp6readily forms high-molecular-weight self-assembled structures as determined by size exclusion chromatography andchemical cross-linking. Negative stain electron microscopy revealed that Drp6 assembles into rings and spirals at physiologicalionic strength. We have also shown that the recombinant Drp6 expressed in bacteria is catalytically active and itsGTPase activity is not enhanced by low salt. These results suggest that, in contrast to dynamins but similar to MxA, Drp6self-assembles in the absence of membrane templates, and its GTPase activity is not affected by ionic strength of the buffer.We discuss the self-assembly structure of Drp6 and explain the basis for lack of membrane-stimulated GTPase activity.

    • 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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