G Protein-Coupled Receptors That Regulate Ion Channels
■ The cardiac muscarinic acetylcholine receptor is a GPCR whose effector protein is a K+ channel. Receptor activation causes release of the Gp7 subunit, which opens K+ channels (see Figure 13-21). The resulting hyperpolariza-tion of the cell membrane slows the rate of heart muscle contraction.
■ Rhodopsin, the photosensitive GPCR in rod cells, comprises the opsin protein linked to 11-cis-retinal. The light-induced isomerization of the 11-cis-retinal moiety produces activated opsin, which then activates the coupled trimeric G protein transducin (Gt) by catalyzing exchange of free GTP for bound GDP on the Gta subunit.
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