Precision vs Flexibility in GPCR signaling
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- 作者：Matthias Elgeti ; Alexander S. Rose ; Franz J. Bartl ; Peter W. Hildebrand ; Klaus-Peter Hofmann ; Martin Heck
- 刊名：The Journal of the American Chemical Society
- 出版年：2013
- 出版时间：August 21, 2013
- 年：2013
- 卷：135
- 期：33
- 页码：12305-12312
- 全文大小：381K
- 年卷期：v.135,no.33(August 21, 2013)
- ISSN：1520-5126
文摘
The G protein coupled receptor (GPCR) rhodopsin activates the heterotrimeric G protein transducin (Gt) to transmit the light signal into retinal rod cells. The rhodopsin activity is virtually zero in the dark and jumps by more than one billion fold after photon capture. Such perfect switching implies both high fidelity and speed of rhodopsin/Gt coupling. We employed Fourier transform infrared (FTIR) spectroscopy and supporting all-atom molecular dynamics (MD) simulations to study the conformational diversity of rhodopsin in membrane environment and extend the static picture provided by the available crystal structures. The FTIR results show how the equilibria of inactive and active protein states of the receptor (so-called metarhodopsin states) are regulated by the highly conserved E(D)RY and Yx7K(R) motives. The MD data identify an intrinsically unstructured cytoplasmic loop region connecting transmembrane helices 5 and 6 (CL3) and show how each protein state is split into conformational substates. The C-termini of the Gt纬- and Gt伪-subunits (G伪CT and G纬CT), prepared as synthetic peptides, are likely to bind sequentially and at different sites of the active receptor. The peptides have different effects on the receptor conformation. While G纬CT stabilizes the active states but preserves CL3 flexibility, G伪CT selectively stabilizes a single conformational substate with largely helical CL3, as it is found in crystal structures. Based on these results we propose a mechanism for the fast and precise signal transfer from rhodopsin to Gt, which assumes a stepwise and mutual reduction of their conformational space. The mechanism relies on conserved amino acids and may therefore underlie GPCR/G protein coupling in general.