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Single cyclic nucleotide-gated channels locked in different ligand-bound states

Nature volume 389, pages 389–392 (1997)Cite this article

Abstract

Cyclic nucleotide-gated (CNG) channels are directly activated by the binding of several ligands1,2,3,4,5,6. For these channels as well as for other allosteric proteins, the functional effects of each ligand-binding event have been difficult to assess because ligands continuously bind and unbind at each site. Furthermore, in retinal rod photoreceptors the low cytoplasmic concentration of cyclic GMP7 means that channels exist primarily in partially liganded states, so it is important to determine how such channels behave. Previous studies of single channels have suggested that they occasionally open to subconducting states at low cGMP2,3,8,9,10, but the significance of these states and how they arise is poorly understood. Here we combine the high resolution of single-channel recording with the use of a photoaffinity analogue of cGMP11,12 that tethers cGMP moieties covalently to their binding sites to show single retinal CNG channels can be effectively locked in four distinct ligand-bound states. Our results indicate that channels open more than they would spontaneously when two ligands are bound (∼1% of the maximum current), significantly more with three ligands bound (∼33%), and open maximally with four ligands bound. In each ligand-bound state, channels opened to two or three different conductance states. These findings place strong constraints on the activation mechanism of CNG channels.

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Figure 1: Covalent attachment of ligands persistently activates channels.
Figure 2: Superimposed cGMP dose–response relations from eight patches, before and after different degrees of covalent ligand attachment (ten.
Figure 3: Subsaturating cGMP concentrations produce subconducting states.
Figure 4: Opening behaviour of partially and fully ligand-bound single channels.

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Acknowledgements

We thank W. Betz, R. L. Brown, Y. Koutalos, A. R. Martin and W. Sather for comments on the manuscript. This work was supported by a grant from the National Eye Institute. ML.R. was the recipient of a National Research Service Award.

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  1. Department of Physiology, Department of Physiology and Biophysics, C-240, University of Colorado School of Medicine, Denver, 80262, Colorado, USA

    MariaLuisa Ruiz & Jeffrey W. Karpen

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  1. MariaLuisa Ruiz
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Correspondence to Jeffrey W. Karpen.

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Ruiz, M., Karpen, J. Single cyclic nucleotide-gated channels locked in different ligand-bound states. Nature 389, 389–392 (1997). https://doi.org/10.1038/38744

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