Ultrastructure and molecular composition of the central filament body in hydrozoan cnidocils

The cnidocils of hydrozoan nematocytes contain modified microtubular axonemes and a cross-striated central filament body. Filament body and axonemal microtubules are interconnected by cross-bridges or are directly attached to one another. Structure and molecular composition of the central filament body was analyzed by means of electron microscopy and immunological techniques. The spindle-like, cross-striated filament body is composed of less than 4 nm thick subfibers. Each unit of its periodical striation pattern is approximately 19 nm broad and comprises subzones with different electron densities. Pure filament bodies are obtainable by a lysis of isolated cnidocils in detergent- and EDTA-containing buffers. The filament body is soluble in 5 mM dithioerythritol (DTE) or 1 M urea. Its structure is stabilized by high concentrations of Na+ or K+ (0.5 M both). Intermediate states of assembly can be induced by 1 mM DTE in lysis buffer which causes the filament bodies to spread into subfibers. CFB43, a monoclonal antibody against filament body protein reacts with a 33 kDa protein in Hydra. Like a polyclonal antibody against SF-assemblin, the protein forming cross-striated system I fibers in unicellular green algae, the antibody cross-reacts with a 36 kDa protein obtained from the unicellular organism Tetrahymena. In immunogold labeling experiments, CFB43 binds to the central filament body in Hydra and recognizes an epitope on the striated rootlets associated with the basal bodies of Tetrahymena. Our results indicate that the central filament body of hydrozoan cnidocils is composed of a protein with properties similar to those of SF-assemblin. In combination with microtubules, the filament body seems to participate in the formation of a stiff mechanical backbone within the cnidocils.