There are no known proteins that catalyze cell fusion in mating, fertilization and organogenesis. In C. elegans mutations in
eff-1 block epithelial cell fusion events.
eff-1 encodes novel type I membrane proteins required for cell fusion (Mohler, Shemer et al., 2002). In the last IWM we showed that EFF-1 is also sufficient to fuse epithelial cells (Suissa et al., 2003). To our knowledge EFF-1 is the only developmental cell fusogen candidate that has been shown to be both necessary and sufficient to fuse cells in vivo (Shemer, Suissa et al., 2004). Moreover, EFF-1 is required and sufficient to fuse most somatic cells in C. elegans (Shemer, Suissa et al., 2004; del Campo et al., 2005). In the dorsal hypodermis of C. elegans a row of 17 cells fuse during embryogenesis. These fusion events are regulated in time and space: While the lateral membrane domains of each cell do not fuse, most anterior and posterior cell membrane domains fuse in wild-type embryos (Podbilewicz and White, 1994). To determine whether different domains of the cell membrane fuse symmetrically, we have analyzed when, where and how fast anterior and posterior membranes fuse within the epidermis of developing animals. We developed a system to simultaneously measure individually fusing epidermal cells in embryos. We showed that each cell membrane domain decides to fuse, or not to fuse, with autonomous and asymmetric kinetics. We measured the kinetics of large expanding gaps, of the order of hundreds of nanometers (up to 2-10 m), resulting from single cell-cell fusions. We found that each fusion event follows sigmoidal kinetics in wild-type and
idf-1 mutant embryos that have Irregular Dorsal Fusion.
idf-1 mutations either block early cell fusion steps or slow down fusion rates. We discovered morphologically stable intermediates of cell fusion characterized by gaps that fail to expand in pharyngeal muscles and hypodermis of
eff-1 mutants. We will discuss cell fusion kinetic maps in the hypodermis and ultrastructural membrane fusion intermediates in two different adult organs in
eff-1(
hy21). We show that EFF-1 proteins are required not only to initiate membrane merger and cytoplasmic mixing, but also to expand and finish syncytia formation.