Balancing G protein selectivity and efficacy in the adenosine A receptor.

The adenosine A receptor (AR) engages several G proteins, notably G and its cognate G protein. This coupling promiscuity is facilitated by a dynamic ensemble, revealed by F nuclear magnetic resonance imaging of AR and G protein. Two transmembrane helix 6 (TM6) activation states, formerly associated with partial and full agonism, accommodate the differing volumes of G and G. While nucleotide depletion biases TM7 toward a fully active state in AR-G, AR-G is characterized by a dynamic inactive/intermediate fraction. Molecular dynamics simulations reveal that the NPxxY motif, a highly conserved switch, establishes a unique configuration in the AR-G complex, failing to stabilize the helix-8 interface with G, and adoption of the active state. The resulting TM7 dynamics hamper G protein coupling, suggesting kinetic gating may be responsible for reduced efficacy in the noncognate G protein complex. Thus, dual TM6 activation states enable greater diversity of coupling partners while TM7 dynamics dictate coupling efficacy.