The yeast atlastin Sey1p may not be sufficient to drive homotypic ER fusion at its physiological concentration

The endoplasmic reticulum (ER) is one of the most important organelle in the cell. The ER has many functions including cytoplasmic calcium level control, protein quality control, and lipid synthesis. These functions are thought to be closely related with the unique structure of the ER, which forms a network of sheets and tubules that are maintained by homotypic ER membrane fusion. Although the exact mechanism and players of homotypic ER fusion is yet to be unveiled, the dynamin-like GTPase atlastin is believed to mediate homotypic ER fusion by forming a homo-dimer in trans between fusing membranes. In this study, we measured the physiological concentration of the yeast atlastin homolog Sey1p from isolated yeast microsomes and performed lipid mixing assays with Sey1p-proteoliposomes bearing various concentrations of Sey1p. Our study revealed that the physiological ratio of lipids to Sey1p molecules was approximately 1.0 x 105 and that lipid mixing was hardly detectable for proteoliposomes bearing Sey1p at ...