The olfactory G protein Gαolf possesses a lower GDP‐affinity and deactivates more rapidly than Gsαshort: consequences for receptor‐coupling and adenylyl cyclase activation

Abstract

The olfactory G protein G_αolf differs from the short splice variant of G_sα (G_sαS) in 80 amino acids, but little is known about biochemical differences between G_αolf and G_sαS. We addressed this question by analyzing fusion proteins of the β₂‐adrenoceptor (β₂AR) and G_αolf and G_sαS, respectively, using Sf9 insect cells as expression system. The fusion ensured defined receptor/G protein stoichiometry and efficient coupling. High‐affinity agonist binding studies showed that G_αolf possesses a lower GDP‐affinity than G_sαS As a result, the agonist‐free β₂AR and the β₂AR occupied by partial agonists were more efficient at promoting GDP‐dissociation from G_αolf than from G_sαS a assessed by guanosine 5’‐O‐(3‐thiotriphosphate) binding, adenylyl cyclase (AC) activity and GTP hydrolysis. Basal AC activity in the absence of GTP was almost sixfold lower in membranes expressing β₂AR‐G_αolf than in membranes expressing β₂AR‐G_sαS at similar levels, reflecting the lower abundance of G_αolf‐GDP relative to G_sαS‐GDP. The maximum agonist‐stimulated AC activity with β₂AR‐G_sαS was more than twofold higher than with β₂AR‐G_αolf, but the relative agonist‐stimulation of AC with β₂AR‐G_αolf was much greater than with β₂AR‐G_sαS. The difference in maximum AC activity can be explained by more rapid deactivation of G_αolf‐GTP by GTP hydrolysis and GTP dissociation relative to G_sαS‐GTP. Taken together, there are biochemical differences between G_αolf and G_sαS, supporting different roles of these G proteins in vivo.