An Intramolecular Contact in Gα Transducin That Participates in Maintaining Its Intrinsic GDP Release Rate

Abstract

Receptor mediated stimulation of the G protein-α subunit leads to exchange of GDP for GTP, activating the protein. Spontaneous GDP release from Gα can also lead to the active state, if GTP in solution binds the nucleotide binding pocket. The purpose of this study is to evaluate the molecular determinants for maintaining the spontaneous GDP release rates between two Gα subunits. Gα_t has a low rate of nucleotide release, compared to Gα_i1. Gα_t/i1 chimeras were used to explore the molecular basis for this behavior. The C-terminal α4-helix, the N-terminal 56 residues and the Switch I/II regions of Gα_t were shown to affect the low spontaneous GDP release rate in Gα_t. A specific molecular contact between Asp26 and Asn191 was found in Gα_t that is not present in Gα_i1. In two chimeras disrupting this interaction produced an increased spontaneous GDP release; restoring the contact present in Gα_t into these chimeras decreased the GDP release rate by half as compared to the original chimeras. Similarly, introduction of this contact in wild-type Gα_i1 decreased the GDP release rate of Gα_i1 by half. Differences in GDP release rates may reflect physiological roles these proteins play in living systems.