A bivalent remipede toxin promotes calcium release via ryanodine receptor activation

Abstract

Multivalent ligands of ion channels have proven to be both very rare and highly valuable in yielding unique insights into channel structure and pharmacology. Here, we describe a bivalent peptide from the venom of Xibalbanus tulumensis, a troglobitic arthropod from the enigmatic class Remipedia, that causes persistent calcium release by activation of ion channels involved in muscle contraction. The high-resolution solution structure of φ-Xibalbin3-Xt3a reveals a tandem repeat arrangement of inhibitor-cysteine knot (ICK) domains previously only found in spider venoms. The individual repeats of Xt3a share sequence similarity with a family of scorpion toxins that target ryanodine receptors (RyR). Single-channel electrophysiology and quantification of released Ca2+ stores within skinned muscle fibers confirm Xt3a as a bivalent RyR modulator. Our results reveal convergent evolution of RyR targeting toxins in remipede and scorpion venoms, while the tandem-ICK repeat architecture is an evolutionary innovation that is convergent with toxins from spider venoms.

Structural Basis of the Bivalency of the TRPV1 Agonist DkTx

“…In the case of DkTx, the peptide‐receptor complex is known, but the resolution of the structure does not reveal structural details of the linker, and a solution structure of the tandem repeat peptide is not available [8–9] . Conversely for Hi1a and Xt3a, the solution structure is known, and details of the inter‐domain orientation and the structure of their linkers are observable in solution, while the peptide‐channel complexes are unknown, making it difficult to conclude how the linker may affect the bivalency of these peptides [3,6] …”

“…[3] Most recently, a tandem repeat ICK peptide, Xt3a, from the venom of a remipede was shown to be a bivalent agonist of ryanodine receptors (RyR). [6] The bivalency of these peptides is remarkable, as simple conjugation of two proteins does not itself necessarily result in enhanced potency and avidity as is evident in some peptide dendrimers. [7] These proteins, therefore, have evolved to simultaneously target distinct receptor sites and are connected by a linker that allows them to achieve this efficiently.…”

“…[8][9] Conversely for Hi1a and Xt3a, the solution structure is known, and details of the inter-domain orientation and the structure of their linkers are observable in solution, while the peptide-channel complexes are unknown, making it difficult to conclude how the linker may affect the bivalency of these peptides. [3,6] In DkTx, the bivalent binding of the peptide has been proposed to follow a sequential binding model, where the linker simply acts as a tether to enhance the local concentration of the second domain, following singledomain binding. [9][10] In this model, the higher potency of DkTx compared to that of the most potent constituent knot (ICK) is difficult to explain, and instead suggests that both domains bind simultaneously.…”

Structural Basis of the Bivalency of the TRPV1 Agonist DkTx

“…Subsequently, a second tandem repeat ICK peptide, Hi1a, from a different spider venom, was also shown to have bivalent action, this time towards acid sensing ion channel 1a (ASIC1a) [3] . Most recently, a tandem repeat ICK peptide, Xt3a, from the venom of a remipede was shown to be a bivalent agonist of ryanodine receptors (RyR) [6] . The bivalency of these peptides is remarkable, as simple conjugation of two proteins does not itself necessarily result in enhanced potency and avidity as is evident in some peptide dendrimers [7] .…”

“…[ 8 , 9 ] Conversely for Hi1a and Xt3a, the solution structure is known, and details of the inter‐domain orientation and the structure of their linkers are observable in solution, while the peptide‐channel complexes are unknown, making it difficult to conclude how the linker may affect the bivalency of these peptides. [ 3 , 6 ]…”