Fig. 8. Model for FER-1-mediated Ca²⁺-dependent MO fusion. MOs are bi-lobed structures comprising a head and membrane-dense body, separated by an electron dense collar. Spermatids await an `activation' signal, with MO heads docked at the PM prior to fusion. Upon receipt of an activation signal, intracellular Ca²⁺ is utilized to relay the signal to be received by FER-1 and by others required for MO fusion. When MOs fuse in spermatozoa, the head and a portion of MO body membrane is incorporated into the PM. A fusion pore remains surrounded by the collar, and the glycoprotein contents are spilled from MOs along the membrane surface of cell body. FER-1 and other proteins can move into the plasma membrane, while the 1CB4 antigen and presumably other proteins remain concentrated within the MO cavity. In fer-1 mutants, the MO heads abut the PM but MOs do not fuse.