TOPOLOGY OF THE PORE-REGION OF A K+ CHANNEL REVEALED BY THE NMR-DERIVED STRUCTURES OF SCORPION TOXINS

The architecture of the pore-region of a voltage-gated K+ channel, Kv1.3, was probed using four high affinity scorpion toxins as molecular calipers. We established the structural relatedness of these toxins by solving the structures of kaliotoxin and margatoxin and comparing them with the published structure of charybdotoxin; a homology model of noxiustoxin was then developed. Complementary mutagenesis of Kv1.3 and these toxins, combined with electrostatic compliance and thermodynamic mutant cycle analyses, allowed us to identify multiple toxin-channel interactions. Our analyses reveal the existence of a shallow vestibule at the external entrance to the pore. This vestibule is similar to 28-32 Angstrom wide at its outer margin, similar to 28-34 Angstrom wide at its base, and similar to 4-8 a deep. The pore is 9-14 Angstrom wide at its external entrance and tapers to a width of 4-5 Angstrom at a depth of similar to 5-7 Angstrom from the vestibule. This structural information should directly aid in developing topological models of the pores of related ion channels and facilitate therapeutic drug design.