Charge conversion enables quantification of the proximity between a normally-neutral mu-conotoxin (GIIIA) site and the Na+ channel pore.
mu-Conotoxin (mu-CTX) inhibits Na+ flux by obstructing the Na+ channel pore. Previous studies of mu-CTX have focused only on charged toxin residues, ignoring the neutral sites. Here we investigated the proximity between the C-terminal neutral alanine (A22) of mu-CTX and the Na+ channel pore by replacing it with the negatively charged glutamate. The analog A22E and wild-type (WT) mu-CTX exhibited identical nuclear magnetic resonance spectra except at the site of replacement, verifying that they have identical backbone structures. A22E significantly reduced mu-CTX affinity for WT mu1 Na+ channels (90-fold), as if the inserted glutamate repels the anionic pore receptor. We then looked for the interacting partner(s) of residue 22 by determining the potency of block of Y401K, Y401A, E758Q, D762K, D762A, E765K, E765A and D1241K channels by WT mu-CTX and A22E, followed by mutant cycle analysis to assess their individual couplings. Our results show that A22E interacts strongly with E765K from domain II (DII) (deltadeltaG=2.2 +/- 0.1 vs. <1 kcal/mol for others). We conclude that mu-CTX residue 22 closely associates with the DII pore in the toxin-bound channel complex. The approach taken may be further exploited to study the proximity of other neutral toxin residues with the Na+ channel pore.
Li
Ronald A
RA
Institute of Molecular Cardiobiology, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
Sato
Kazuki
K
Kodama
Kyoko
K
Kohno
Toshiyuki
T
Xue
Tian
T
Tomaselli
Gordon F
GF
Marbán
Eduardo
E
D000409:Alanine / Q000235:genetics / Q000378:metabolism; D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D001665:Binding Sites; D020916:Conotoxins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D004594:Electrophysiology; D018698:Glutamic Acid / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D008958:Models, Molecular; D018482:Muscle, Skeletal / Q000378:metabolism; D016297:Mutagenesis, Site-Directed / Q000235:genetics; D019906:Nuclear Magnetic Resonance, Biomolecular; D018408:Patch-Clamp Techniques; D011485:Protein Binding; D011487:Protein Conformation; D051381:Rats; D026941:Sodium Channel Blockers; D015222:Sodium Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D055672:Static Electricity
D020916:Conotoxins; D026941:Sodium Channel Blockers; D015222:Sodium Channels; C046101:conotoxin GIII; D018698:Glutamic Acid; D000409:Alanine