The adhesive disc is a highly complex apparatus that allows mobilid ciliates to attach to the tissues of a variety of aquatic invertebrates and vertebrates. The disc comprises concentric rings of rigid skeletal pieces interconnected by filamentous material. This study explored the biochemical properties of the filamentous disc material in the trichodinid Trichodina pediculus. Calcium sensitivity of this material was suggested in vitro by the appearance of transverse cross-striation along bundles of filaments following calcium shock, and complete solubilization of the filamentous material in the presence of EGTA. A 23-kDa immunoanalog of centrins was immunoprecipitated from the EGTA extract. The protein binds calcium as indicated by (45) Ca(2+) blot overlay and Ca(2+) -induced shifts in electrophoretic mobility. Using Ca(2+) /EGTA buffers, we demonstrated a direct relationship between extraction of the filaments and solubilization of the protein. Immunofluorescence and immunoelectron microscopy confirmed that the protein localized to the filamentous disc material and revealed cross-reactivity with the spasmoneme, which is the prototype of ion-sensitive, centrin-like contractile systems in ciliates. The possibility that the filamentous disc material may be a novel example of Ca(2+) -sensitive, centrin-based systems found in ciliates is discussed.