*Review* **Fish Cytolysins in All Their Complexity**

**Fabiana V. Campos 1, Helena B. Fiorotti 1,2, Juliana B. Coitinho <sup>1</sup> and Suely G. Figueiredo 1,\***


**Abstract:** The majority of the effects observed upon envenomation by scorpaenoid fish species can be reproduced by the cytolysins present in their venoms. Fish cytolysins are multifunctional proteins that elicit lethal, cytolytic, cardiovascular, inflammatory, nociceptive, and neuromuscular activities, representing a novel class of protein toxins. These large proteins (MW 150–320 kDa) are composed by two different subunits, termed α and β, with about 700 amino acid residues each, being usually active in oligomeric form. There is a high degree of similarity between the primary sequences of cytolysins from different fish species. This suggests these molecules share similar mechanisms of action, which, at least regarding the cytolytic activity, has been proved to involve pore formation. Although the remaining components of fish venoms have interesting biological activities, fish cytolysins stand out because of their multifunctional nature and their ability to reproduce the main events of envenomation on their own. Considerable knowledge about fish cytolysins has been accumulated over the years, although there remains much to be unveiled. In this review, we compiled and compared the current information on the biochemical aspects and pharmacological activities of fish cytolysins, going over their structures, activities, mechanisms of action, and perspectives for the future.

**Keywords:** fish venoms; cytolysins; multifunctionality; pore formation

**Key Contribution:** In this review, we discussed the current knowledge on structural and physiopharmacological properties of fish cytolysins, the multifunctional toxins responsible for the major symptoms observed upon envenomation by fish. In addition, we attempted to provide some insights into unknown aspects of their multi-mechanistic mode of action, which may be useful for future studies on these interesting molecules.
