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Biochemistry. 1996 Mar 19;35(11):3503-7.

Importance of highly conserved anionic residues and electrostatic interactions in the activity and structure of the cardiotonic polypeptide anthopleurin B.

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Department of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati College of Medicine, Ohio 45267-0524, USA.


Several polypeptide toxins from sea anemones caused delayed inactivation of mammalian voltage-dependent sodium channels, resulting in a positive inotropic effect on the heart. Anthopleurin B (ApB), a toxin produced by the sea anemone Anthopleura xanthogrammica, is the most potent of all known anemone toxins. Previous studies in this laboratory have both defined and revealed an important role for the cationic cluster of Arg-12, Arg-14, and Lys-49 in the expression of ApB's biological activity. In the present investigation, we explore the role of all remaining charged residues by producing and characterizing mutants of ApB at Asp-7, Asp-9, Lys-37, His-39, and His-34. Recombinant toxins have been purified to homogeneity and their abilities to enhance veratridine-dependent sodium uptake in cell lines expressing either the neuronal or cardiac isoform of the sodium channel evaluated. Replacement of Asp-7 results in a product that fails to fold, while muteins H39A and H34A have activities very similar or identical to wild-type ApB. In contrast, the D9N and K37A muteins are 7-12-fold less active that wild-type ApB, and truncation of the side chain in D9A results in a further decrease in activity, especially in the cardiac model. We conclude that although a negative charge per se is not essential at position 9, the presence of a hydrogen-bond forming side chain is critical both for appropriate folding and for interaction with the sodium channel. Because the K37A and H39A mutant toxins can fold normally, neither Lys-37 nor His-39 seem to participate in an intramolecular salt bridge, in contrast to suggestions arising from NMR studies of ApA and ApB. However, Lys-37 may play a role in channel interaction.

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