Tunable pKa values and the basis of opposite charge selectivities in nicotinic-type receptors

Abstract

Among ion channels, only the nicotinic-receptor superfamily has evolved to generate both cation- and anion-selective members. Although other, structurally unrelated, neurotransmitter-gated cation channels exist, no other type of neurotransmitter-gated anion channel, and thus no other source of fast synaptic inhibitory signals, has been described so far. In addition to the seemingly straightforward electrostatic effect of the presence (in the cation-selective members) or absence (in the anion-selective ones) of a ring of pore-facing carboxylates, mutational studies have identified other features of the amino-acid sequence near the intracellular end of the pore-lining transmembrane segments (M2) that are also required to achieve the high charge selectivity shown by native channels1,2,3,4,5,6,7,8,9,10. However, the mechanism underlying this more subtle effect has remained elusive11 and a subject of speculation. Here we show, using single-channel electrophysiological recordings to estimate the protonation state of native ionizable side chains, that anion-selective-type sequences favour whereas cation-selective-type sequences prevent the protonation of the conserved, buried basic residues at the intracellular entrance of the pore (the M2 0′ position). We conclude that the previously unrecognized tunable charge state of the 0′ ring of buried basic side chains is an essential feature of these channels’ versatile charge-selectivity filter.

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Figure 1: The versatile charge selectivity of nicotinic-type receptors.
Figure 2: A proline mutation unveils a proton-binding site.
Figure 3: The side chain of the 0′ basic residue is the proton-binding site.
Figure 4: Not only prolines, not only insertions.

Change history

  • 23 June 2011

    Fig. 1a was corrected.

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Acknowledgements

We thank S. Sine for wild-type muscle AChR cDNA; M. Slaughter and D. Papke for wild-type α1 GlyR cDNA; S. Elenes for critical advice on fast-perfusion experiments; E. Jakobsson and H. Robertson for discussions; and G. Papke, M. Maybaum, J. Pizarek and C. Staehlin for technical assistance. This work was supported by a grant from the US National Institutes of Health (R01-NS042169 to C.G.).

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Correspondence to Claudio Grosman.

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This file contains Supplementary Text, Supplementary Figures 1-10 with legends, Supplementary Tables 1-4 and additional references. (PDF 2047 kb)

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Cymes, G., Grosman, C. Tunable pKa values and the basis of opposite charge selectivities in nicotinic-type receptors. Nature 474, 526–530 (2011). https://doi.org/10.1038/nature10015

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