Subtype specificity of scorpion beta-toxin Tz1 interaction with voltage-gated sodium channels is determined by the pore loop of domain 3.

Voltage-gated sodium (Nav) channels are modulated by a variety of specific neurotoxins. Scorpion beta-toxins affect the voltage-dependence of channel gating: In their presence, Nav channels activate at subthreshold membrane voltages. Previous mutagenesis studies have revealed that the beta-toxin Css4 interacts with the extracellular linker between segments 3 and 4 in domain 2 of Nav channels with the effect to trap this voltage sensor in an open position (Neuron 21: 919-931, 1998 ). The voltage sensor of domain 2 was thus identified to constitute a major part of neurotoxin receptor site 4. In this work, we studied the effects of the beta-toxin Tz1 from the Venezuelan scorpion Tityus zulianus on various mammalian Nav channel types expressed in HEK 293 cells. Although skeletal muscle channels (Nav1.4) were strongly affected by Tz1, the neuronal channels Nav1.6 and Nav1.2 were less sensitive, and the cardiac Nav1.5 and the peripheral nerve channel Nav1.7 were essentially insensitive. Analysis of channel chimeras in which whole domains of Nav1.2 were inserted into a Nav1.4 background revealed that the Nav1.2 phenotype was not conferred to Nav1.4 by domain 2 but by domain 3. The interaction epitope could be narrowed down to residues Glu1251, Lys1252, and His1257 located in the C-terminal pore loop in domain 3. The receptor site for beta-toxin interaction with Nav channels thus spans domains 2 and 3, where the pore loop in domain 3 specifies the pharmacological properties of individual neuronal Nav channel types.