Dimerization of the class A G protein-coupled neurotensin receDtor NTS1 alters G protein interaction

G protein-coupled receptors (GPCRs) have been found as monomers but also as dimers or higher-order oligomers in cells. The relevance of the monomeric or dimeric receptor state for G protein activation is currently under debate for class A rhodopsin-like GPCRs. Clarification of this issue requires the availability of well defined receptor preparations as monomers or dimers and an assessment of their ligand-binding and G protein-coupling properties. We show by pharmacological and hydrodynamic experiments that purified neurotensin receptor NTS1, a class A GPCR, dimerizes in detergent solution in a concentration-dependent manner, with an apparent affinity in the low nanomolar range. At low receptor concentrations, NTS1 binds the agonist neurotensin with a Hill slope of approximate to 1; at higher receptor concentrations, neurotensin binding displays positive cooperativity with a Hill slope of approximate to 2. NTS1 monomers activate GaqB(1 gamma 2), whereas receptor dimers catalyze nucleotide exchange with lower affinity. Our results demonstrate that NTS1 dimerization per se is not a prerequisite for G protein activation.