Repair of hair bundles in sea anemones by secreted proteins.

Sea anemones are sessile invertebrates that detect movements of prey using numerous hair bundles located on tentacles surrounding their mouth. Previously we found that hair bundles of anemones are structurally and functionally similar to those of vertebrates. After 10-15 min exposure to calcium depleted buffers, hair bundles in chickens suffer moderate damage from which they recover in 12 h without requiring new protein synthesis [Zhao, Yamoah and Gillespie, Proc. Natl. Acad. Sci. USA 94 (1996) 15469-15474]. We find that after 1 h exposure to calcium free seawater, hair bundles of anemones suffer extensive damage from which they recover in 4 h, apparently because of newly synthesized, secretory proteins called 'repair proteins'. Recovery is delayed in a dose dependent fashion by cycloheximide. In the presence of exogenously added repair proteins, recovery occurs within 8 min and is cycloheximide insensitive. Recovery is ascertained by a bioassay performed on intact specimens, by electrophysiology, and by timelapse video microscopy. Fraction beta, a chromatographic fraction with bioactivity comparable to the complete mixture of repair proteins, consists of complexes having an estimated mass of 2000 kDa. Avidin based cytochemistry suggests that biotinylated fraction beta binds to damaged hair bundles. SDS-PAGE gel electrophoresis demonstrates that fraction beta contains 8-10 polypeptides of 90 kDa or smaller. At least four of these polypeptides apparently are consumed during the repair process. Negatively stained samples of fraction beta are shown by transmission electron microscopy to include filamentous structures similar in length (150 nm) and width (6 nm) to linkages between stereocilia. The filamentous structures can be associated with globular structures (20 nm in diameter). A model is presented wherein repair proteins comprise replacement linkages and enzymes that attach linkages to appropriate membrane proteins.