Raftophilic rhodopsin-clusters offer stochastic platforms for G protein signalling in retinal discs

Fumio Hayashi,Kenichi Morigaki,Yasushi Tanimoto,Keisuke Okada,Keiji Seno,Natsumi Saito,Shohei Maekawa

doi:10.1038/s42003-019-0459-6

Fumio Hayashi, Kenichi Morigaki + Show 5 more

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https://doi.org/10.1038/s42003-019-0459-6

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Rhodopsin is a G protein-coupled receptor (GPCR) that initiates the phototransduction cascade in retinal disc membrane. Recent studies have suggested that rhodopsin forms highly ordered rows of dimers responsible for single-photon detection by rod photoreceptors. Dimerization is also known to confer to rhodopsin a high affinity for ordered lipids (raftophilicity). However, the role of rhodopsin organization and its raftophilicity in phototransduction remains obscure, owing to the lack of direct observation of rhodopsin dynamics and distribution in native discs. Here, we explore the single-molecule and semi-multimolecule behaviour of rhodopsin in native discs. Rhodopsin forms transient meso-scale clusters, even in darkness, which are loosely confined to the disc centre. Cognate G protein transducin co-distributes with rhodopsin, and exhibits lateral translocation to the disc periphery upon activation. We demonstrate that rhodopsin offers inherently distributed and stochastic platforms for G protein signalling by self-organizing raftophilic clusters, which continually repeat generation/extinction in the disc membrane.

Highlights

Rhodopsin is a G protein-coupled receptor (GPCR) that initiates the phototransduction cascade in retinal disc membrane
We and others have already found that 10–30% of rhodopsin in the dark-adapted disc membrane is recovered in the detergent-resistant membrane (DRM)[28,29], the distribution of which is quite a useful index of raftophilicity for membrane proteins[30]
Our results clearly show that rhodopsin forms transient meso-sized raftophilic clusters, loosely confined in the disc membrane, which are excluded from the raftophobic disc periphery

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Introduction

Rhodopsin is a G protein-coupled receptor (GPCR) that initiates the phototransduction cascade in retinal disc membrane. A cryo-electron tomographic study has shown that at least ten rhodopsin dimers form pairs of rows (tracks) aligned parallel to the disc incisures[10], and their accompanying simulation results suggested that the track structure can explain the uniform single-photon response in rod photoreceptors, a long-standing question in phototransduction studies[16]. Such structural studies have provided static pictures of the supramolecular structure of rhodopsin, a coarsegrained molecular-dynamics simulation study implied that the rhodopsin organization would be formed through relatively weak (1.2–3.6 kcal/mol) protein−protein interactions, via multiple dimerization interfaces, and that the organization should be transient[17]. Our subsequent study revealed that the Gt-stabilized dimer of rhodopsin is responsible for the raftophilicity of the Rh*–Gt complex, and that palmitoyl modification of rhodopsin is a prerequisite for raftophilicity attained by dimerization[8]

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