Is Proteolytic Cleavage Essential for the Enhanced Activity of Hydra Pore-Forming Toxin, HALT-4?

Hydra actinoporin-like toxin 4 (HALT-4) differs from other actinoporins due to its N-terminal propart that contains approximately 103 additional residues. Within this region, we identified five dibasic residues and assumed that, when cleaved, they could potentially exhibit HALT-4's cytolytic activity. We created five truncated versions of HALT-4 (tKK1, tKK2, tRK3, tKK4 and tKK5) to investigate the role of the N-terminal region and potential cleavage sites on the cytolytic activity of HALT-4. However, our results demonstrated that the propart-containing HALT-4 (proHALT-4), as well as the truncated versions tKK1 and tKK2, exhibited similar cytolytic activity against HeLa cells. In contrast, tRK3, tKK4 and tKK5 failed to kill HeLa cells, indicating that cleavage at the KK1 or KK2 sites did not enhance cytolytic activity but may instead facilitate the sorting of tKK1 and tKK2 to the regulated secretory pathway for eventual deposition in nematocysts. Moreover, RK3, KK4 and KK5 were unlikely to serve as proteolytic cleavage sites, as the amino acids between KK2 and RK3 are also crucial for pore formation.