Title: Tarantula Toxins use Common Surfaces for Interacting with Kv and ASIC Ion Channels
Abstract: Tarantula toxins such as Hanatoxin and GxTx1E target voltage-activated potassium (Kv) channels by partitioning into the lipid membrane and binding to their voltage-sensing domains to modify gating. While aspects of this inhibitory mechanism have been well-studied, no structures of the toxin-channel complex have thus far been solved. Recently, an acid sensing ion channel (ASIC) was crystalized in complex with PcTx1, a structurally related tarantula toxin that modifies activation of the channel by protons. That X-ray structure shows that the PcTx1 binding site is localized within the extracellular domain well above the membrane, where the toxin clamps onto helix-5 and inserts an Arg-finger into the subunit interface. Here we compare membrane interactions and channel binding surfaces of GxTx1E and PcTx1 to understand the relationship between these two classes of tarantula toxins. Our results demonstrate that PcTx1 can interact with membranes, albeit more weakly and superficially compared to toxins targeting voltage sensors. Chimeras in which helix-5 of ASIC replaced the S3b helix of Kv2.1 retained voltage-dependent gating, but did not transfer sensitivity to PcTx1. Remarkably, alanine-scanning mutagenesis of GxTx1E reveals that residues involved in binding to Kv channels overlap extensively with those PcTx1 uses to bind to ASIC. We conclude that tarantula toxins have evolved to use related surfaces to bind to structurally unrelated ion channels even when the detailed molecular interactions and physical environments are distinct.