ULTRASTRUCTURE AND MOLECULAR COMPOSITION OF THE CENTRAL FILAMENT BODYIN HYDROZOAN CNIDOCILS

The cnidocils of hydrozoan nematocytes contain modified microtubular a xonemes and a cross-striated central filament body. Filament body and axonemal microtubules are interconnected by cross-bridges or are direc tly attached to one another. Structure and molecular composition of th e central filament body was analyzed by means of electron microscopy a nd 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 comprise s subzones with different electron densities. Pure filament bodies are obtainable by a lysis of isolated cnidocils in detergent- and EDTA-co ntaining buffers. The filament body is soluble in 5 mM dithioerythrito l (DTE) or 1 M urea. Its structure is stabilized by high concentration s of Na+ or K+ (0.5 M both). Intermediate states of assembly can be in duced by 1 mM DTE in lysis buffer which causes the filament bodies to spread into subfibers. CFB43, a monoclonal antibody against filament b ody protein reacts with a 33 kDa protein in Hydra, Like a polyclonal a ntibody against SF-assemblin, the protein forming cross-striated syste m I fibers in unicellular green algae, the antibody cross-reacts with a 36 kDa protein obtained from the unicellular organism Tetrahymena. I n immunogold labeling experiments, CFB43 binds to the central filament body in Hydra and recognizes an epitope on the striated rootlets asso ciated with the basal bodies of Tetrahymena. Our results indicate that the central filament body of hydrozoan cnidocils is composed of a pro tein with properties similar to those of SF-assemblin. In combination with microtubules, the filament body seems to participate in the forma tion of a stiff mechanical backbone within the cnidocils.