Localization of proline receptors involved in regulating nematocyst discharge

At least two distinct classes of chemoreceptor are known to regulate nematocyst discharge in sea anemones. One class of receptor binds N‐acetylated sugars and the other class binds certain amino compounds including the imino acid, proline. Previously, receptors for N‐acetylated sugars were localized using colloidal gold conjugates of bovine submaxillary mucin. The sugar receptors are known to cycle by receptor‐mediated endocytosis from the apical surface of supporting cells. In this study, colloidal gold conjugates of polyproline were used to determine the distribution of proline receptors. We find that polyproline‐gold, like mucin‐gold, localizes to the apical surface of supporting cells. Binding of polyproline‐gold to supporting cells is blocked by excess free polyproline or free proline. After appearing at the cell surface, polyproline‐gold enters endosomes, followed by multivesicular bodies. To test for possible artifact due to colloidal gold, trafficking of cationized ferritin (CF) was monitored in the presence of proline. CF appears at the cell surface, followed by endosomes, multivesicular bodies, and lysosomes. Some evidence is presented that proline receptors are sorted from N‐acetylated sugar receptors in endosomes. We propose that receptor‐mediated endocytosis functions in anemone tentacles to enhance the temporal resolution of chemosensing.