Restricting mobility of Gsα relative to the β2-adrenoceptor enhances adenylate cyclase activity by reducing Gsα GTPase activity

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Abstract
The β2-adrenoceptor (β2AR) activates the G-protein Gsα to stimulate adenylate cyclase (AC). Fusion of the β2AR C-terminus to the N-terminus of Gsα (producing β2ARGsα) markedly increases the efficiency of receptor/G-protein coupling compared with the non-fused state. This increase in coupling efficiency can be attributed to the physical proximity of receptor and G-protein. To determine the optimal length for the tether between receptor and G-protein we constructed fusion proteins from which 26 [β2AR(Δ26)Gsα] or 70 [β2AR(Δ70)Gsα] residues of the β2AR C-terminus had been deleted and compared the properties of these fusion proteins with the previously described β2ARGsα. Compared with β2ARGsα, basal and agonist-stimulated GTP hydrolysis was markedly decreased in β2AR(Δ70)Gsα, whereas the effect of the deletion on binding of guanosine 5´-[γ-thio]triphosphate (GTP[S]) was relatively small. Surprisingly, deletions did not alter the efficiency of coupling of the β2AR to Gsα as assessed by GTP[S]-sensitive high-affinity agonist binding. Moreover, basal and ligand-regulated AC activities in membranes expressing β2AR(Δ70)Gsα and β2AR(Δ26)Gsα were higher than in membranes expressing β2ARGsα. These findings suggest that restricting the mobility of Gsα relative to the β2AR results in a decrease in G-protein inactivation by GTP hydrolysis and thereby enhanced activation of AC.

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