title: Structure and action of heteronemertine polypeptide toxins: importance of amphipathic helix for activity of Cerebratulus lacteus toxin A-III.
abstract: The marine heteronemertine Cerebratulus lacteus produces a family of protein cytolysins designated as A-toxins. Limited subtilisin digests of the most abundant homolog, toxin A-III, yield two major products which may be purified by high-performance liquid chromatography. One product is shown to represent residues 1-86 and the other contains the entire toxin sequence (1-95). Both polypeptides are shown to lack internal protease nicks. The 1-95 polypeptide retains full cytolytic activity in comparison to native toxin, whereas 1-86 has an activity that is approximately four times lower. Extensive treatment of A-III with carboxypeptidase Y yields a polypeptide containing residues 1-75 which is totally devoid of hemolytic activity. Residues 63-95 of native A-III have been predicted to form a relatively hydrophobic alpha-helix which is potentially important for activity. The circular dichroism spectrum of 1-95 is in excellent agreement with both experimental and Chou-Fasman-predicted secondary structures of native A-III, while the spectra of 1-86 and 1-75 indicate a loss of helicity quantitatively consistent with the removal of residues 87-95 and 76-95, respectively. Combined with our earlier data on bilayer penetration by N-terminal sequences (K. M. Blumenthal (1982) Biochemistry 21, 4229-4233], the current results indicate a direct involvement of both ends of A-III in lytic activity. The C-terminal region may function by contributing a membrane binding site in the form of an amphipathic helix.
mesh_terms: D000596:Amino Acids / Q000032:analysis; D000818:Animals; D055598:Chemical Phenomena; D002621:Chemistry; D006461:Hemolysis / Q000187:drug effects; D006801:Humans; D066298:In Vitro Techniques; D008387:Marine Toxins / Q000633:toxicity; D010446:Peptide Fragments / Q000032:analysis; D010447:Peptide Hydrolases; D010986:Platyhelminths; D011487:Protein Conformation; D013381:Subtilisins
chemical_list: D000596:Amino Acids; C026601:Cerebratulus lacteus toxin A-III; D008387:Marine Toxins; D010446:Peptide Fragments; D010447:Peptide Hydrolases; D013381:Subtilisins