title: Structure of the SthK carboxy-terminal region reveals a gating mechanism for cyclic nucleotide-modulated ion channels.
abstract: Cyclic nucleotide-sensitive ion channels are molecular pores that open in response to cAMP or cGMP, which are universal second messengers. Binding of a cyclic nucleotide to the carboxyterminal cyclic nucleotide binding domain (CNBD) of these channels is thought to cause a conformational change that promotes channel opening. The C-linker domain, which connects the channel pore to this CNBD, plays an important role in coupling ligand binding to channel opening. Current structural insight into this mechanism mainly derives from X-ray crystal structures of the C-linker/CNBD from hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels. However, these structures reveal little to no conformational changes upon comparison of the ligand-bound and unbound form. In this study, we take advantage of a recently identified prokaryote ion channel, SthK, which has functional properties that strongly resemble cyclic nucleotide-gated (CNG) channels and is activated by cAMP, but not by cGMP. We determined X-ray crystal structures of the C-linker/CNBD of SthK in the presence of cAMP or cGMP. We observe that the structure in complex with cGMP, which is an antagonist, is similar to previously determined HCN channel structures. In contrast, the structure in complex with cAMP, which is an agonist, is in a more open conformation. We observe that the CNBD makes an outward swinging movement, which is accompanied by an opening of the C-linker. This conformation mirrors the open gate structures of the Kv1.2 channel or MthK channel, which suggests that the cAMP-bound C-linker/CNBD from SthK represents an activated conformation. These results provide a structural framework for better understanding cyclic nucleotide modulation of ion channels, including HCN and CNG channels.
authors.affiliation: Laboratory of Structural Neurobiology, KU Leuven, Herestraat 49, PB601, Leuven, B-3000, Belgium.
authors.forename: Divya
authors.initials: D
authors.lastname: Kesters
authors.affiliation: Laboratory of Structural Neurobiology, KU Leuven, Herestraat 49, PB601, Leuven, B-3000, Belgium.
authors.forename: Marijke
authors.initials: M
authors.lastname: Brams
authors.affiliation: Laboratory of Structural Neurobiology, KU Leuven, Herestraat 49, PB601, Leuven, B-3000, Belgium.
authors.forename: Mieke
authors.initials: M
authors.lastname: Nys
authors.affiliation: Laboratory of Structural Neurobiology, KU Leuven, Herestraat 49, PB601, Leuven, B-3000, Belgium.
authors.forename: Eveline
authors.initials: E
authors.lastname: Wijckmans
authors.affiliation: Laboratory of Structural Neurobiology, KU Leuven, Herestraat 49, PB601, Leuven, B-3000, Belgium.
authors.forename: Radovan
authors.initials: R
authors.lastname: Spurny
authors.affiliation: Laboratory of Ion Channel Research, KU Leuven, Herestraat 49, PB802, Leuven, B-3000, Belgium.
authors.forename: Thomas
authors.initials: T
authors.lastname: Voets
authors.affiliation: Laboratory of Toxicology and Pharmacology, KU Leuven, Herestraat 49, PB922, Leuven, B-3000, Belgium.
authors.forename: Jan
authors.initials: J
authors.lastname: Tytgat
authors.affiliation: University Hospital Jena, Institute of Physiologie II, Kollegiengasse 9, 07743, Jena, Germany.
authors.forename: Jana
authors.initials: J
authors.lastname: Kusch
authors.affiliation: Laboratory of Structural Neurobiology, KU Leuven, Herestraat 49, PB601, Leuven, B-3000, Belgium.
authors.forename: Chris
authors.initials: C
authors.lastname: Ulens
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D018360:Crystallography, X-Ray; D000242:Cyclic AMP / Q000737:chemistry*; D006152:Cyclic GMP / Q000737:chemistry*; D015640:Ion Channel Gating; D008958:Models, Molecular; D015221:Potassium Channels / Q000737:chemistry*; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013142:Spirochaeta
chemical_list: D001426:Bacterial Proteins; D015221:Potassium Channels; D000242:Cyclic AMP; D006152:Cyclic GMP