Structural implications of weak Ca2+ block in Drosophila cyclic nucleotide-gated channels.
Bottom Line: 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.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.
Affiliation: Department of Physiology and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390.Show MeSH
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Mentions: In the absence of a CNG channel structure, insight into the structural details underlying ion nonselectivity and Ca2+ block has been limited to K+ channel models (Doyle et al., 1998; Zhou et al., 2001; Long et al., 2007) and more recently the prokaryotic nonselective cation channel NaK from Bacillus cereus (Shi et al., 2006; Alam et al., 2007). These bacterial channels have a topology very similar to CNG channel pore domain, even though they lack the cyclic nucleotide–binding domain. By using NaK as the model system, we have engineered a set of CNG-mimicking NaK chimeras, NaK2CNG, in which the NaK selectivity filter sequence was replaced with those of canonical CNG channels, and we determined their structures to high resolution (Derebe et al., 2011). In one such chimera named NaK2CNG-E, the NaK filter sequence of TVGDGNFSP was replaced with TVGETPPP to simulate the most commonly seen CNG channel pores (Fig. 1). The resulting mutant faithfully recapitulated the selectivity and Ca2+-blocking property of canonical CNG channels, and the Ca2+-blocking sensitivity can be similarly modulated by changing the glutamate residues in the filter to aspartate (NaK2CNG-D) or asparagine (NaK2CNG-N) (Derebe et al., 2011).
Affiliation: Department of Physiology and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390.