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Asymmetric properties of rod cGMP Phosphodiesterase 6 (PDE6): structural and functional analysis

Photoreceptor cGMP Phsophodiesterase 6 (PDE6) is the effector molecule of visual signal transduction and mediates fast response of light signals. The rod holo-PDE6 comprises catalytic (α, β; each ~ 90 kDa) and two identical inhibitiory (γ; ~ 10 kDa) subunits. The catalytic subunits comprise N-terminal tandem GAF domains followed by C-terminal catalytic domains and isoprenylations for membrane-association. Contrary to activation of other tandem GAF comprising PDEs, PDE6 activation does not occur via cGMP-induced concerted conformational changes. Rather two copies of the α-subunit of retinal G-Protein (Gα*), transducin, activate PDE6 by partially displacing the inhibitory subunits. The activation of PDE6 has therefore been described as a “de-inhibition”. The affinity of Gα* to PDE6 and the enzymatic activity of the intermediary 1:1 complex is highly disputed, therefore a conclusive activation model is lacking so far.

Our combined structural, enzymatic and computational investigations deal with the activation-mechanism of PDE6. Our cryo electron microscopy (EM) structure of PDEαβ catalytic core shows an elongated bell-shaped structure with symmetric side-by-side arrangement of the two subunits with flexible membrane-binding domains. A comparison with nearly full-length inactive PDE2A structure [1] suggests that less compaction of both subunits and higher degree of conformational freedom of the catalytic domains result in constitutive activation of PDE6αβ, which is kept inactive by the inhibitory γ subunits. Furthermore, the structure of PDE6 suggests Gα* binding-sites pointing to opposing faces. The enzymatic characterization using Gα* titration of the PDE6 however reveal striking asymmetry of the two catalytic subunits with a high and a low affinity binding site for Gα*. Occupancy of the PDE6 with one Gα* induces negligible activity, whereas occupancy with two copies of Gα* leads to full enzyme activity. Such an activation mechanism constitutes a “coincidence switch” that allows noise filtering (i.e., spontaneously produced Gα* do not activate PDE6). Our spatiotemporal simulation work indeed confirms that spontaneously generated Gα* lead to the formation of singly occupied PDE6 and only a high local concentration of Gα*, as produced by an active receptor (rhodopsin), leads to doubly Gα* occupied effector complex. Therefore the localized large concentration of Gα* combined with the asymmetric properties of PDE6 constitutes a “density switch” that allows suppression effector level noise and reliable reporting of single quantum events in rod photoreceptor cells.

References

  1. 1.

    Pandit J, Forman MD, Fennell KF, Dillman KS, Menniti FS: Mechanism for the allosteric regulation of phosphodiesterase 2A deduced from the X-ray structure of a near full-length construct. PNAS. 2009, 106: 18225-18230. 10.1073/pnas.0907635106.

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Correspondence to Bilal M Qureshi.

Additional information

Bilal M Qureshi, Elmar Behrmann contributed equally to this work.

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This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Keywords

  • Cryo Electron Microscopy
  • PDE6 Activation
  • cGMP Phosphodiesterase
  • Asymmetric Property
  • Full Enzyme Activity