Genetic approach to regulated exocytosis using functional complementation in Paramecium: identification of the ND7 gene required for membrane fusion.

Paramecium is a unicellular organism that possesses a specialized pathway for regulated secretion that is amenable to genetic studies. Numerous mutations affecting the process have been isolated over the years, among which is a subclass blocking the terminal step of fusion of the secretory granule with the plasma membrane. We report herein the cloning by functional complementation of one such gene, ND7. The 506-amino acid polypeptide encoded by ND7 is predicted to be a type I integral membrane protein with a highly charged cytosolic domain featuring amphipathic and coiled-coil regions. This structure is compatible with the physiological data on the mutant nd7-1 suggesting that the protein is anchored in the membrane of the secretory granule and that it may interact with other proteins. This work presents the first identification by a genetic approach of a novel gene involved in regulated secretion and establishes Paramecium as a powerful model system for the genetic dissection of this process.