A Cation-Ï Interaction Discriminates among Sodium Channels That Are Either Sensitive or Resistant to Tetrodotoxin Block
Voltage-gated sodium channels control the upstroke of the action potential in excitable cells of nerve and muscle tissue, making them ideal targets for exogenous toxins that aim to squelch electrical excitability. One such toxin, tetrodotoxin (TTX), blocks sodium channels with nanomolar affinity onl...
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Published in | The Journal of biological chemistry Vol. 282; no. 11; p. 8044 |
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Main Authors | , , , , |
Format | Journal Article |
Language | English |
Published |
American Society for Biochemistry and Molecular Biology
16.03.2007
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Online Access | Get full text |
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Summary: | Voltage-gated sodium channels control the upstroke of the action potential in excitable cells of nerve and muscle tissue,
making them ideal targets for exogenous toxins that aim to squelch electrical excitability. One such toxin, tetrodotoxin (TTX),
blocks sodium channels with nanomolar affinity only when an aromatic Phe or Tyr residue is present at a specific location
in the external vestibule of the ion-conducting pore. To test whether TTX is attracted to Tyr 401 of Na V 1.4 through a cation-Ï interaction, this aromatic residue was replaced with fluorinated derivatives of Phe using in vivo nonsense suppression. Consistent with a cation-Ï interaction, increased fluorination of Phe 401 , which reduces the negative electrostatic potential on the aromatic face, caused a monotonic increase in the inhibitory constant
for block. Trifluorination of the aromatic ring decreased TTX affinity by â¼50-fold, a reduction similar to that caused by
replacement with the comparably hydrophobic residue Leu. Furthermore, we show that an energetically equivalent cation-Ï interaction
underlies both use-dependent and tonic block by TTX. Our results are supported by high level ab initio quantum mechanical calculations applied to a model of TTX binding to benzene. Our analysis suggests that the aromatic side
chain faces the permeation pathway where it orients TTX optimally and interacts with permeant ions. These results are the
first of their kind to show the incorporation of unnatural amino acids into a voltage-gated sodium channel and demonstrate
that a cation-Ï interaction is responsible for the obligate nature of an aromatic at this position in TTX-sensitive sodium
channels. |
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ISSN: | 0021-9258 1083-351X |
DOI: | 10.1074/jbc.M611334200 |