A granulin fold arising from a common ICK cysteine framework

What is it about?

Venomous marine cone snails produce peptide toxins (conotoxins) that bind ion channels and receptors with high specificity and therefore are important pharmacological tools. Conotoxins contain conserved cysteine residues that form disulfide bonds that stabilize their structures. To gain structural insight into the large, yet poorly characterized conotoxin H-superfamily, we used NMR and CD spectroscopy along with MS-based analyses to investigate H-Vc7.2 from Conus victoriae, a peptide with a VI/VII cysteine framework. This framework has CysI–CysIV/CysII– CysV/CysIII–CysVI connectivities, which have invariably been associated with the inhibitor cystine knot (ICK) fold. However, the solution structure of recombinantly expressed and purified H-Vc7.2 revealed that although it displays the expected cysteine connectivities, H-Vc7.2 adopts a different fold consisting of two stacked β-hairpins with opposing β-strands connected by two parallel disulfide bonds, a structure homologous to the N-terminal region of the human granulin protein. Using structural comparisons, we subsequently identified several toxins and nontoxin proteins with this “mini-granulin” fold.

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Why is it important?

The work demonstrates that, despite having the disulfide pattern expected of a peptide with a VI/VII cysteine framework, H-Vc7.2 did not adopt an inbitor cystine know (ICK) fold. Rather the peptide displayed a mini-granulin fold, a structure comprised of two short, stacked β-hairpins connected by two parallel disulfide bonds. Whereas the same cysteine framework can give rise to different cysteine connectivities, the finding that the same cysteine connectivities can dictate different folds is unusual. This work therefore emphasizes that caution must be taken in assuming a specific structure based on a certain cysteine framework, or even a known disulfide pattern.

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