Receptor-mediated endocytosis of a chemoreceptor involved in triggering the discharge of cnidae in a sea anemone tentacle

Collodial gold coated with the glycoprotein, bovine submaxillary mucin (BSM-gold), was used to localize chemoreceptors known to be involved in triggering the discharge of cnidae in sea anemones. BSM-gold binds exclusively at the apical surface of the supporting cell, the cell adjacent to the cnidocyte (Watson and Hessinger, 1986). Subsequent to binding, BSM-gold is internalized into endosomes and then translocated to multivesicular bodies (MVBs) and lysosomes. At cold temperature (4 °C), BSM-gold appears in endosomes near the surface of the cell but not in endosomes located more medially in the cell, nor in MVBs or lysosomes. The kinetics and sequence of intracellular translocation of BSM-gold were studied by fixing animals at various intervals following incubation in BSM-gold. Unlike that for supporting cells adjacent to non-cnidocytes, the amount of gold at the surface of supporting cells adjacent to penetrant cnidocytes does not seem to change despite considerable internalization of the mucin-probe. Apparently, free receptors replace receptor-ligand complexes in a one-for-one fashion in these cells.