Potassium channel modulation by a toxin domain in matrix metalloprotease

Peptide toxins found in a wide array of venoms block K+ channels, causing profound physiological and pathological effects. Here we describe the first functional K+ channel-blocking toxin domain in a mammalian protein. MMP23 (matrix metalloprotease 23) contains a domain (MMP23TxD) that is evolutionarily related to peptide toxins from sea anemones. MMP23TxD shows close structural similarity to the sea anemone toxins BgK and ShK. Moreover, this domain blocks K+ channels in the nanomolar to low micromolar range (Kv1.6 > Kv1.3 > Kv1.1 = Kv3.2 > Kv1.4, in decreasing order of potency) while sparing other K+ channels (Kv1.2, Kv1.5, Kv1.7, and KCa3.1). Full-length MMP23 suppresses K+ channels by co-localizing with and trapping MMP23TxD-sensitive channels in the ER. Our results provide clues to the structure and function of the vast family of proteins that contain domains related to sea anemone toxins. Evolutionary pressure to maintain a channel-modulatory function may contribute to the conservation of this domain throughout the plant and animal kingdoms.

A helical conotoxin from Conus imperialis has a novel cysteine framework and defines a new superfamily

Cone snail venoms are a rich source of peptides, many of which are potent and selective modulators of ion channels and receptors. Here we report the isolation and characterization of two novel conotoxins from the venom of Conus imperialis. These two toxins contain a novel cysteine framework, C-C-C-CC-C, which has not been found in other conotoxins described to date. We name it framework XXIII and designate the two toxins im23a and im23b; cDNAs of these toxins exhibit a novel signal peptide sequence, which defines a new K-superfamily. The disulfide connectivity of im23a has been mapped by chemical mapping of partially reduced intermediates and by NMR structure calculations, both of which establish a I-II, III-IV, V-VI pattern of disulfide bridges. This pattern was also confirmed by synthesis of im23a with orthogonal protection of individual cysteine residues. The solution structure of im23a reveals that im23a adopts a novel helical hairpin fold. A cluster of acidic residues on the surface of the molecule is able to bind calcium. The biological activity of the native and recombinant peptides was tested by injection into mice intracranially and intravenously to assess the effects on the central and peripheral nervous systems, respectively. Intracranial injection of im23a or im23b into mice induced excitatory symptoms; however, the biological target of these new toxins has yet to be identified.