Is proteolytic cleavage essential for the activation of Hydra pore-forming toxin, HALT-4?

The mature form of Hydra actinoporin-like toxin 4 (mHALT-4) differs from other actinoporins primarily by bearing approximately 115 additional residues at the N-terminus. Five dibasic residues were found in this extended region and we assume that, when cleaved, each truncated HALT-4 (tKK1, tKK2, tRK3, tKK4 and tKK5) could exhibit an enhanced cytolytic activity. However, our results showed that mHALT-4, tKK1 and tKK2 possessed similar cytolytic activity against HeLa cells, whereas tRK3, tKK4 and tKK5 failed to kill HeLa cells. Therefore, the cleavage of KK1 or KK2 sites did not enhance the cytolytic activity of tKK1 and tKK2 but might facilitate the sorting of tKK1 and tKK2 to the regulated secretory pathway and eventually deposit them in the nematocyst. In contrast, RK3, KK4 and KK5 were unlikely to serve as the proteolytic cleavage sites since the amino acids between KK2 and RK3 are also crucial for the pore formation. HighlightsO_LIFive dibasic cleavage sites are identified at the N-terminal region of HALT-4 but they are unlikely to have a role in the enhancement of HALT-4 cytotoxicity. C_LIO_LIThe first two dibasic sites, KK1 and KK2, may be involved in sorting HALT-4 to the regulated secretory pathway where HALT-4 can be transported to the nematocyst. C_LI