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Visualizing Activation of a GPCR
Activation and signaling by G protein–coupled receptors (GPCRs), which can function as monomers or dimers, occur through conformational changes. Hlavackova et al. introduced fluorescent proteins and epitope tags into and engineered the C terminus of metabotropic glutamate receptor 1 (mGluR1) to produce chimeric proteins useful for the analysis of the inter- and intrasubunit conformational changes associated with activation of this dimeric GPCR. Intrasubunit rearrangements triggered a decrease in fluorescence resonance energy transfer (FRET) between fluorophores within the same subunit, and intersubunit rearrangements triggered an increase in FRET between fluorophores on different subunits. Kinetic analysis showed that intersubunit rearrangements occurred faster than did intrasubunit rearrangements. Furthermore, only one subunit in each dimer adopted an active conformation. Thus, for a dimeric GPCR, ligand binding first causes intersubunit movements, which are followed by an intrasubunit conformational change that puts one subunit in an active state.
