Mapping the site of block by tetrodotoxin and saxitoxin of sodium channel II.
The SS2 and adjacent regions of the 4 internal repeats of sodium channel II were subjected to single mutations involving, mainly, charged amino acid residues. These sodium channel mutants, expressed in Xenopus oocytes by microinjection of cDNA-derived mRNAs, were tested for sensitivity to tetrodotoxin and saxitoxin and for single-channel conductance. The results obtained show that mutations involving 2 clusters of predominantly negatively charged residues, located at equivalent positions in the SS2 segment of the 4 repeats, strongly reduce toxin sensitivity, whereas mutations of adjacent residues exert much smaller or no effects. This suggests that the 2 clusters of residues, probably forming ring structures, take part in the extracellular mouth and/or the pore wall of the sodium channel. This view is further supported by our finding that all mutations reducing net negative charge in these amino acid clusters cause a marked decrease in single-channel conductance.
Terlau
H
H
Max-Planck-Institut für biophysikalische Chemie, Göttingen, Germany.
Heinemann
S H
SH
Stühmer
W
W
Pusch
M
M
Conti
F
F
Imoto
K
K
Numa
S
S
D000200:Action Potentials / Q000187:drug effects; D000595:Amino Acid Sequence; D000818:Animals; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010449:Peptide Mapping; D051381:Rats; D012530:Saxitoxin / Q000235:genetics* / Q000494:pharmacology; D015222:Sodium Channels / Q000737:chemistry* / Q000187:drug effects; D013779:Tetrodotoxin / Q000235:genetics* / Q000494:pharmacology
D015222:Sodium Channels; D012530:Saxitoxin; D013779:Tetrodotoxin