Actinoporins are a family of pore-forming toxins produced by sea anemones as part of their venomous cocktail. These proteins remain soluble and stably folded in aqueous solution, but when interacting with sphingomyelin-containing lipid membranes, they become integral oligomeric membrane structures that form a pore permeable to cations, which leads to cell death by osmotic shock. Actinoporins appear as multigenic families within the genome of sea anemones: several genes encoding very similar actinoporins are detected within the same species. The Caribbean Sea anemone Stichodactyla helianthus produces three actinoporins (sticholysins I, II and III; StnI, StnII and StnIII) that differ in their toxic potency. For example, StnII is about four-fold more effective than StnI against sheep erythrocytes in causing hemolysis, and both show synergy. However, StnIII, recently discovered in the S. helianthus transcriptome, has not been characterized so far. Here we describe StnIII's spectroscopic and functional properties and show its potential to interact with the other Stns. StnIII seems to maintain the well-preserved fold of all actinoporins, characterized by a high content of β-sheet, but it is significantly less thermostable. Its functional characterization shows that the critical concentration needed to form active pores is higher than for either StnI or StnII, suggesting differences in behavior when oligomerizing on membrane surfaces. Our results show that StnIII is an interesting and unexpected piece in the puzzle of how this Caribbean Sea anemone species modulates its venomous activity.

放线菌素是海葵作为有毒鸡尾酒的一部分产生的成孔毒素家族。这些蛋白质在水溶液中保持可溶并稳定折叠，但是当与含鞘磷脂的脂质膜相互作用时，它们变成完整的寡聚膜结构，形成可渗透阳离子的孔，导致渗透压导致细胞死亡。放线菌素作为海葵基因组中的多基因家族出现：在同一物种中检测到几种编码非常相似的放线菌素的基因。加勒比海葵Stichodactyla helianthus产生三种放线菌素（sticholysins I，II和III； StnI，StnII和StnIII），它们的毒力不同。例如，在引起溶血方面，StnII对绵羊红细胞的效力比StnI大四倍，并且两者均显示出协同作用。然而，最近发现于向日葵中的StnIII转录组，到目前为止尚未鉴定。在这里，我们描述了StnIII的光谱和功能特性，并显示了其与其他Stns相互作用的潜力。StnIII似乎保持了所有放线菌素的保存良好的折叠，其特征在于高含量的β-折叠，但其热稳定性明显较低。它的功能特性表明，形成活性孔所需的临界浓度高于StnI或StnII，这表明在膜表面进行低聚时，其行为存在差异。我们的研究结果表明，StnIII是一个有趣且出乎意料的发现，它困扰着这个加勒比海海葵物种如何调节其有毒活性。