GENETIC-ANALYSIS OF MYOBLAST FUSION - BLOWN FUSE IS REQUIRED FOR PROGRESSION BEYOND THE PREFUSION COMPLEX

The events of myoblast fusion in Drosophila are dissected here by comb ining genetic analysis with light and electron microscopy. We describe a new and essential intermediate step in the process, the formation o f a prefusion complex consisting of ''paired vesicles.'' These pairs o f vesicles from different cells align with each other across apposed p lasma membranes. This prefusion complex resolves into dense membrane p laques between apposed cells; these cells then establish cytoplasmic c ontinuity by fusion of small areas of plasma membrane followed by vesi culation of apposed membranes. Different steps in this process are spe cifically blocked by mutations in four genes required for myoblast fus ion. One of these genes, blown fuse, encodes a novel cytoplasmic prote in expressed in unfused myoblasts that is essential for progression be yond the prefusion complex stage.