Characterization of a Cyclic Nucleotide-Activated K super(+) Channel and its Lipid Environment by Using Solid-State NMR Spectroscopy

Voltage-gated ion channels are large tetrameric multidomain membrane proteins that play crucial roles in various cellular transduction pathways. Because of their large size and domain-related mobility, structural characterization has proved challenging. We analyzed high-resolution solid-state NMR data on different isotope-labeled protein constructs of a bacterial cyclic nucleotide-activated K super(+) channel (MlCNG) in lipid bilayers. We could identify the different subdomains of the 4355 residue protein, such as the voltage-sensing domain and the cyclic nucleotide binding domain. Comparison to ssNMR data obtained on isotope-labeled cell membranes suggests a tight association of negatively charged lipids to the channel. We detected spectroscopic polymorphism that extends beyond the ligand binding site, and the corresponding protein segments have been associated with mutant channel types in eukaryotic systems. These findings illustrate the potential of ssNMR for structural investigations on large membrane-embedded proteins, even in the presence of local disorder. Size does matter: By using solid-state NMR spectroscopy (ssNMR), we have structurally characterized a 4355 residue channel, its lipid environment, and polymorphic states that might be relevant to function. This study highlights the potential of ssNMR to characterize large membrane proteins and their environments in native lipid conditions.