Editors' ChoiceBiochemistry

Visualizing G protein activation

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Science Signaling  03 Sep 2019:
Vol. 12, Issue 597, eaaz3175
DOI: 10.1126/scisignal.aaz3175

Structures provide insights into the mechanism by which the GPCR rhodopsin activates the G protein transducin.

The G protein–coupled receptor (GPCR) rhodopsin is found in rod outer segment membranes in the retina and is composed of the apoprotein opsin covalently bound to the ligand 11-cis retinal. Exposure to light converts the ligand to all-trans retinal, which activates the receptor. Rhodopsin then couples to the G protein transducin (Gt), inducing GDP-GTP exchange at the G protein α-subunit (Gαt), which then dissociates from the Gβγ dimer to initiate downstream signaling. Gao et al. isolated rhodopsin from bovine rod outer segment membranes and mixed it with Gβ1γ1 and an engineered α-subunit, eGαt, in the presence or absence of the nanobody Nb35*, which binds to G proteins to help with structure determination. These complexes were capable of undergoing nucleotide exchange and G protein activation. The authors then used single-particle cryo-EM to solve the structures of the Nb35*-bound and Nb35*-free complexes. These structures revealed the extent of the interface between the receptor and the G protein, as well as the contacts that are required for activated rhodopsin to induce nucleotide exchange at Gαt, which has a higher affinity for GDP than that of other G protein α-subunits. Comparison between nanobody-bound and -free structures suggested that nanobodies may interfere with the separation between the α-subunit and the βγ dimer upon receptor activation, an effect that may need to be kept in mind when interpreting the structures of other nanobody-bound G proteins. Lastly, the structures revealed a previously unappreciated role for the Gβ subunit in stabilizing the helical domain of Gαt and promoting nucleotide exchange. Together, these findings increase our understanding of G protein activation by rhodopsin and suggest a role for Gβγ in promoting receptor-induced nucleotide exchange at the α-subunit.

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