Structural studies of ion permeation and Ca2+ blockage of a bacterial channel mimicking the cyclic nucleotide-gated channel pore

Department of Physiology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9040, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 01/2011; 108(2):592-7. DOI: 10.1073/pnas.1013643108
Source: PubMed


Cyclic nucleotide-gated (CNG) channels play an essential role in the visual and olfactory sensory systems and are ubiquitous in eukaryotes. Details of their underlying ion selectivity properties are still not fully understood and are a matter of debate in the absence of high-resolution structures. To reveal the structural mechanism of ion selectivity in CNG channels, particularly their Ca(2+) blockage property, we engineered a set of mimics of CNG channel pores for both structural and functional analysis. The mimics faithfully represent the CNG channels they are modeled after, permeate Na(+) and K(+) equally well, and exhibit the same Ca(2+) blockage and permeation properties. Their high-resolution structures reveal a hitherto unseen selectivity filter architecture comprising three contiguous ion binding sites in which Na(+) and K(+) bind with different ion-ligand geometries. Our structural analysis reveals that the conserved acidic residue in the filter is essential for Ca(2+) binding but not through direct ion chelation as in the currently accepted view. Furthermore, structural insight from our CNG mimics allows us to pinpoint equivalent interactions in CNG channels through structure-based mutagenesis that have previously not been predicted using NaK or K(+) channel models.

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    • "At present, the crystal structure of CNG channels has not yet been solved, but mimics of their filter region are available. Indeed, a set of chimeric NaK channels mimicking CNG channels – generated by replacing the NaK selectivity filter sequence with those of CNG – have been constructed and their crystal structure solved (Derebe et al. 2011b). The selectivity filter of NaK2CNG-E chimera adopts an architecture intermediate between that of NaK and KcsA pores and contains only three contiguous ion binding sites equivalent to sites S 2 –S 4 of the KcsA (Fig. 1C). "
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