Helix dipole movement and conformational variability contribute to allosteric GDP release in Gα_i subunits

Heterotrimeric G proteins (Gαβγ) transmit signals from activated G protein-coupled receptors (GPCRs) to downstream effectors through a guanine nucleotide signaling cycle. Numerous studies indicate that the carboxy-terminal α5 helix of Gα subunits participates in α-receptor binding, and previous EPR studies suggest this receptor-mediated interaction induces a rotation and translation of the α5 helix of the Gα subunit [Oldham, W. M., et al. (2006) Nat. Struct. Mol. Biol. 13, 772-777]. On the basis of this result, an engineered disulfide bond was designed to constrain the α5 helix of Gα_il into its EPR-measured receptor-associated conformation through the introduction of cysteines at position 56 in the α1 helix and position 333 in the α5 helix (I56C/Q333C Gα_il). A functional mimetic of the EPR-measured α5 helix dipole movement upon receptor association was additionally created by introduction of a positive charge at the amino terminus of this helix, D328R Gα_il. Both proteins exhibit a dramatically elevated level of basal nucleotide exchange. The 2.9 Å resolution crystal structure of I56C/Q333C Gα_il in complex with GDP-AlF ₄^- reveals the shift of the α5 helix toward the guanine nucleotide binding site that is anticipated by EPR measurements. The structure of the I56C/Q333C Gα_il subunit further revealed altered positions for the switch regions and throughout the Gα _il subunit, accompanied by significantly elevated crystallographic temperature factors. Combined with previous evidence in the literature, the structural analysis supports the critical role of electrostatics of the α5 helix dipole and overall conformational variability during nucleotide release.