Abstract
The second extracellular loop (EL2) of rhodopsin forms a cap over the binding site of its photoreactive 11-cis retinylidene chromophore. A crucial question has been whether EL2 forms a reversible gate that opens upon activation or acts as a rigid barrier. Distance measurements using solid-state 13C NMR spectroscopy between the retinal chromophore and the β4 strand of EL2 show that the loop is displaced from the retinal binding site upon activation, and there is a rearrangement in the hydrogen-bonding networks connecting EL2 with the extracellular ends of transmembrane helices H4, H5 and H6. NMR measurements further reveal that structural changes in EL2 are coupled to the motion of helix H5 and breaking of the ionic lock that regulates activation. These results provide a comprehensive view of how retinal isomerization triggers helix motion and activation in this prototypical G protein–coupled receptor.
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Acknowledgements
This work was supported by the US National Insitutes of Health (NIH)–National Science Foundation instrumentation grants (S10 RR13889 and DBI-9977553), a grant from the NIH to S.O.S. (GM-41412), and a grant from the US-Israel Binational Science Foundation to M.S. We thank C.A. Opefi for help with the M288A and M288L mutants and gratefully acknowledge the W.M. Keck Foundation for support of the NMR facilities in the Center of Structural Biology at Stony Brook. M.S. acknowledges support from the Kimmelman Center for Biomolecular Structure and Assembly.
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Ahuja, S., Hornak, V., Yan, E. et al. Helix movement is coupled to displacement of the second extracellular loop in rhodopsin activation. Nat Struct Mol Biol 16, 168–175 (2009). https://doi.org/10.1038/nsmb.1549
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DOI: https://doi.org/10.1038/nsmb.1549
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