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Structural basis for nucleotide exchange in heterotrimeric G proteins

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Science  19 Jun 2015:
Vol. 348, Issue 6241, pp. 1361-1365
DOI: 10.1126/science.aaa5264

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How a receptor transmits a signal

G protein–coupled receptors (GPCRs) transmit diverse external signals into the cell. When activated by an outside stimulus, they bind to a G protein inside the cell and accelerate exchange of a bound guanosine diphosphate (GDP) nucleotide for guanosine triphosphate, which initiates intercellular signaling. Dror et al. used atomic-level molecular dynamics simulations to show how GPCRs enhance GDP release. The G protein is dynamic and frequently adopts a conformation that exposes GDP even without the receptor bound. GPCR binding to this conformation favors an additional structural rearrangement that favors GDP release. The authors confirmed these predictions experimentally using double electron-electron resonance spectroscopy.

Science, this issue p. 1361

Abstract

G protein–coupled receptors (GPCRs) relay diverse extracellular signals into cells by catalyzing nucleotide release from heterotrimeric G proteins, but the mechanism underlying this quintessential molecular signaling event has remained unclear. Here we use atomic-level simulations to elucidate the nucleotide-release mechanism. We find that the G protein α subunit Ras and helical domains—previously observed to separate widely upon receptor binding to expose the nucleotide-binding site—separate spontaneously and frequently even in the absence of a receptor. Domain separation is necessary but not sufficient for rapid nucleotide release. Rather, receptors catalyze nucleotide release by favoring an internal structural rearrangement of the Ras domain that weakens its nucleotide affinity. We use double electron-electron resonance spectroscopy and protein engineering to confirm predictions of our computationally determined mechanism.

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