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 path way for regulated secretion that is amenable to genetic studies. Numer ous 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 clon ing 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 amph ipathic 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 int eract with other proteins. This work presents the first identification by a genetic approach of, a novel gene involved in regulated secretio n and establishes Paramecium as a powerful model system for the geneti c dissection of this process.