Structural implications of weak Ca2+ block in Drosophila cyclic nucleotide-gated channels

Yee Ling Lam, Weizhong Zeng, Mehabaw Getahun Derebe, Youxing Jiang

Research output: Contribution to journalArticle

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Abstract

Calcium permeability and the concomitant calcium block of monovalent ion current ('Ca2+ 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 Ca2+ block, the degree of block displayed by different CNG channels varies greatly. For instance, the Drosophila melanogaster CNG channel shows only weak Ca2+ 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 Ca2+ block properties of the parent CNG channels. Here, we used the same strategy to determine the structural basis of the weak Ca2+ 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 Ca2+ block compared with other NaK2CNG chimeras. Moreover, mutating the corresponding threonine (T318) to proline in Drosophila CNG channels increased Ca2+ 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 Ca2+ block.

Original languageEnglish (US)
Pages (from-to)255-263
Number of pages9
JournalJournal of General Physiology
Volume146
Issue number3
DOIs
StatePublished - Sep 1 2015

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Cyclic Nucleotide-Gated Cation Channels
Threonine
Proline
Glutamic Acid
Ions
Calcium
Drosophila melanogaster
Drosophila
Drosophila Cng protein
Cations
Permeability
Signal Transduction

ASJC Scopus subject areas

  • Physiology

Cite this

Structural implications of weak Ca2+ block in Drosophila cyclic nucleotide-gated channels. / Lam, Yee Ling; Zeng, Weizhong; Derebe, Mehabaw Getahun; Jiang, Youxing.

In: Journal of General Physiology, Vol. 146, No. 3, 01.09.2015, p. 255-263.

Research output: Contribution to journalArticle

Lam, Yee Ling ; Zeng, Weizhong ; Derebe, Mehabaw Getahun ; Jiang, Youxing. / Structural implications of weak Ca2+ block in Drosophila cyclic nucleotide-gated channels. In: Journal of General Physiology. 2015 ; Vol. 146, No. 3. pp. 255-263.
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AB - Calcium permeability and the concomitant calcium block of monovalent ion current ('Ca2+ 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 Ca2+ block, the degree of block displayed by different CNG channels varies greatly. For instance, the Drosophila melanogaster CNG channel shows only weak Ca2+ 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 Ca2+ block properties of the parent CNG channels. Here, we used the same strategy to determine the structural basis of the weak Ca2+ 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 Ca2+ block compared with other NaK2CNG chimeras. Moreover, mutating the corresponding threonine (T318) to proline in Drosophila CNG channels increased Ca2+ 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 Ca2+ block.

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