It takes two transducins to activate the cGMP-phosphodiesterase 6 in retinal rods.

Bilal M Qureshi,Jan Giesebrecht,Elmar Behrmann,Martin Heck,Trevor D Lamb,Jörg Bürger,Klaus Peter Hofmann,Johannes Schöneberg,Christian M T Spahn,Frank Noé,Thorsten Mielke,Justus Loerke

doi:10.1098/rsob.180075

Bilal M Qureshi, Jan Giesebrecht + Show 10 more

Open Access

https://doi.org/10.1098/rsob.180075

Copy DOI

Abstract

Among cyclic nucleotide phosphodiesterases (PDEs), PDE6 is unique in serving as an effector enzyme in G protein-coupled signal transduction. In retinal rods and cones, PDE6 is membrane-bound and activated to hydrolyse its substrate, cGMP, by binding of two active G protein α-subunits (Gα*). To investigate the activation mechanism of mammalian rod PDE6, we have collected functional and structural data, and analysed them by reaction–diffusion simulations. Gα* titration of membrane-bound PDE6 reveals a strong functional asymmetry of the enzyme with respect to the affinity of Gα* for its two binding sites on membrane-bound PDE6 and the enzymatic activity of the intermediary 1 : 1 Gα* · PDE6 complex. Employing cGMP and its 8-bromo analogue as substrates, we find that Gα* · PDE6 forms with high affinity but has virtually no cGMP hydrolytic activity. To fully activate PDE6, it takes a second copy of Gα* which binds with lower affinity, forming Gα* · PDE6 · Gα*. Reaction–diffusion simulations show that the functional asymmetry of membrane-bound PDE6 constitutes a coincidence switch and explains the lack of G protein-related noise in visual signal transduction. The high local concentration of Gα* generated by a light-activated rhodopsin molecule efficiently activates PDE6, whereas the low density of spontaneously activated Gα* fails to activate the effector enzyme.

Highlights

Cyclic nucleotide phosphodiesterase enzymes (PDEs) play important regulatory roles in diverse signal transduction cascades by degrading the second messenger cyclic nucleotides cAMP and cGMP [1,2,3]
The holo-phosphodiesterase 6 (PDE6) (PDE6abgg) comprises two catalytic cGMP-binding sites that are activated by Ga* binding to two regulatory sites
The resulting dose–response curves consistently reveal that Ga* has a very low affinity for soluble PDE6

Summary

Introduction

Cyclic nucleotide phosphodiesterase enzymes (PDEs) play important regulatory roles in diverse signal transduction cascades by degrading the second messenger cyclic nucleotides cAMP and cGMP [1,2,3]. PDEs have a role in the recovery phase of signal transduction cascades [5], but phosphodiesterase 6 (PDE6) acts as the activating effector of visual signal transduction in retinal rods and cones. The active form of rhodopsin (R*) catalyses the exchange of bound GDP for GTP in many copies of the heterotrimeric G-protein transducin (G). The rapid degradation of cGMP by active PDE6 causes the closure of cGMP-gated channels, membrane hyperpolarization and neuronal response [6,7]. G-protein and effector are all associated with the membranes

Methods

Results

Conclusion