Sequential¹H-NMR assignments of neurotoxin III from the sea anemone Heteractis macrodactylus and structural comparison with related toxins

The complete sequence-specific assignment of resonances in the¹H-NMR spectrum of the polypeptide neurotoxin III (Hm III) from the sea anemone Heteractis macrodactylus is described. Comparison of the chemical shifts and pattern of NOEs for Hm III with those for the related toxin Hp III from Heteractis paumotensis, which differs only in the substitution of Asn for Tyr at position 11, shows that the overall secondary and tertiary structures are conserved. The largest differences in chemical shift caused by the substitution at position 11 are observed for the NH resonances of Arg-13, Thr-14, Ala-15, Leu-17, and Cys-26. The C^αH resonances influenced most are those of ASP-6, Gly-9, Leu-17, and Glu-42, while the most affected C^βH resonances are from Leu-17, Glu-28, and Lys-32. The absence of long-range NOEs to the aromatic ring of Tyr-11 as well as the lack of significant chemical shift effects on residues outside the loop comprising residues 7-16 confirm that this part of the loop makes no long-lived contacts with the rest of the molecule. The deviations from random coil shifts of Hm III are compared with those of the related anemone toxins Hp II, Hp III, and toxin I from Stichodactyla helianthus (Sh I). The similarity in deviations in chemical shift as a function of sequence position for these four toxins emphasizes the overall structural homology among these polypeptides.