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Structural implications of weak Ca(2+) block in Drosophila cyclic nucleotide–gated channels
Calcium permeability and the concomitant calcium block of monovalent ion current (“Ca(2+) block”) are properties of cyclic nucleotide–gated (CNG) channel fundamental to visual and olfactory signal transduction. Although most CNG channels bear a conserved glutamate residue crucial for Ca(2+) block, t...
Autores principales: | , , , |
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Formato: | Online Artículo Texto |
Lenguaje: | English |
Publicado: |
The Rockefeller University Press
2015
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Materias: | |
Acceso en línea: | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4555469/ https://www.ncbi.nlm.nih.gov/pubmed/26283200 http://dx.doi.org/10.1085/jgp.201511431 |
Sumario: | Calcium permeability and the concomitant calcium block of monovalent ion current (“Ca(2+) block”) are properties of cyclic nucleotide–gated (CNG) channel fundamental to visual and olfactory signal transduction. Although most CNG channels bear a conserved glutamate residue crucial for Ca(2+) block, the degree of block displayed by different CNG channels varies greatly. For instance, the Drosophila melanogaster CNG channel shows only weak Ca(2+) block despite the presence of this glutamate. We previously constructed a series of chimeric channels in which we replaced the selectivity filter of the bacterial nonselective cation channel NaK with a set of CNG channel filter sequences and determined that the resulting NaK2CNG chimeras displayed the ion selectivity and Ca(2+) block properties of the parent CNG channels. Here, we used the same strategy to determine the structural basis of the weak Ca(2+) block observed in the Drosophila CNG channel. The selectivity filter of the Drosophila CNG channel is similar to that of most other CNG channels except that it has a threonine at residue 318 instead of a proline. We constructed a NaK chimera, which we called NaK2CNG-Dm, which contained the Drosophila selectivity filter sequence. The high resolution structure of NaK2CNG-Dm revealed a filter structure different from those of NaK and all other previously investigated NaK2CNG chimeric channels. Consistent with this structural difference, functional studies of the NaK2CNG-Dm chimeric channel demonstrated a loss of Ca(2+) block compared with other NaK2CNG chimeras. Moreover, mutating the corresponding threonine (T318) to proline in Drosophila CNG channels increased Ca(2+) block by 16 times. These results imply that a simple replacement of a threonine for a proline in Drosophila CNG channels has likely given rise to a distinct selectivity filter conformation that results in weak Ca(2+) block. |
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