Gi and Gs have a structurally different sub unit in their alpha chain.
The receptors for PGE1 and adenosine interact with inhibitory Gi,
which contains the same β and γ subunits as stimulatory Gs but a
different α subunit (Giα). In response to binding of an inhibitory
ligand to its receptor, the associated Gi protein releases its bound
GDP and binds GTP; the active Giα · GTP complex then dissociates from
Gβγ and inhibits (rather than stimulates) adenylyl cyclase.
Both β1- and β2-adrenergic receptors are coupled to G proteins (Gs), which activate adenylyl cyclase. In contrast, α1 and α2 receptors are coupled to two other G proteins, Gq and Gi, respectively. Gi inhibits adenylyl cyclase, and Gq stimulates phospholipase C to generate IP3 and DAG as second messengers.
A comparison of chimeric receptor 1, which interacts with Gs, and
chimeric receptor 3, which interacts with Gi, shows that the G protein
specificity is determined primarily by the source of the
cytosol-facing loop between α helices 5 and 6. A comparison of
chimeras 1 and 2 indicates that α helix 7 plays a role in determining
the ligand-binding . B. Kobilka et al., 1988, Science 240:1310; W. A.
Catterall, 1989, Science 243:236.
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