Sh. Heinemann et al., MOLECULAR PORE STRUCTURE OF VOLTAGE-GATED SODIUM AND CALCIUM CHANNELS, Brazilian journal of medical and biological research, 27(12), 1994, pp. 2781-2802
1. Voltage-gated ion channels have a high homology in their primary st
ructure but yet show quite distinct ion selection properties. Therefor
e, ion selection was studied in voltage-gated wild-type and mutant sod
ium (Na RBII) and calcium (Ca BI) channels, which were heterologously
expressed in Xenopus oocytes, using two-electrode voltage clamp and pa
tch clamp methods. 2. In order to facilitate the electrophysiological
experiments, fast sodium channel inactivation was abolished by introdu
cing the point mutation F1489Q into the linker between the homologous
repeats III and IV. 3. The ion pore in rat brain sodium channel II was
located by testing single-point mutants for their sensitivity to exte
rnally applied tetrodotoxin (TTX), leading to a model in which part of
the linker S5-S6 folds into the membrane to form at least part of the
ion pore; two amino acids from each domain are thereby of major impor
tance for TTX association and ion permeation, presumably forming two r
ings of predominantly charged residues in three-dimensional space (out
er ring: E387, E945, M1425, D1717; inner ring: D384, E942, K1422, A171
4). 4. When the residue F385 in repeat I of Na RBII channel is mutated
to a cysteine, the residue being at the homologous position in cardia
c sodium channels, the channel lost its high sensitivity for TTX and g
ained a high sensitivity for external Cd2+ and Zn2+, properties found
in cardiac sodium channels. Mutation of the other cysteine in the pore
region, C940, had no effect on channel block by TTX or by external di
valent cations. 5. The inner ring in sodium channels constitutes a sel
ectivity filter: after mutation of residues K1422 and A1714 into gluta
mates, the homologous residues in calcium channels, the sodium channel
s show ion selection properties similar to calcium channels: a) no sel
ectivity among monovalent cations, b) strong block of monovalent curre
nt by divalent cations, c) permeation of divalent cations. 6. Reverse
mutations in the BI calcium channel, where glutamate residues were mut
ated into neutral glutamines, showed that also in this channel these a
mino acids form a selectivity filter. The individual residues interact
differently with the permeating ions suggesting an asymmetric spatial
arrangement of the four pore-forming regions of the ion channel pen.