Separation and characterization of four different amino acid sequence variants of a sea anemone (Stichodactyla helianthus) protein cytolysin.

A basic protein cytolysin previously isolated from the Caribbean sea anemone Stichodactyla helianthus was shown by CM cellulose chromatography to consist of four isotoxins possessing different N-terminal amino acid sequences. These are designated as toxins I-IV in order of increasing isoelectric point. The estimated molecular sizes (17,400-18,200) of toxins I-III were very similar; toxins I and II posses one additional amino acid at their amino terminus relative to toxin III. Under denaturing conditions, toxin IV behaved as a significantly larger (19,600) polypeptide; Edman sequencing established that it possesses a seven residue extension at the N-terminal end relative to toxin III. None of the variants contained half-cystines or reducing sugars. Toxin III contributed 83% of the total purified cytolytic (hemolytic) activity, toxin II 14%, and the relatively insoluble toxins I and IV together only contributed about 3% of the total cytolytic activity. Cytolysin III lysed Ehrlich ascitic tumour cells, but when administered intraperitoneally in nonlethal doses to mice already inoculated with this tumour, it failed to protect the mice against the tumour. Comparison of the partial amino acid sequence of equinatoxin, another sea anemone protein cytolysin, with that of Stichodactyla cytolysin III indicates they are highly homologous. Many other cytolytic proteins isolated from sea anemones share these properties with Stichodactyla cytolysins: (1) selective inhibition of hemolytic activity by preincubation with sphingomyelin, (2) a molecular size of 10,000-20,000, and (3) an isoelectric point of 9 or above.