Mutational Analysis of G Protein α Subunit G_oα Expressed in Escherichia coli
G protein-mediated signal transduction is dependent on alpha subunit interactions with βy subunits, receptors, effectors, magnesium ions, and guanine nucleotides. The interdependence of these interactions can be probed by mutational analysis. We developed large scale screening procedures in recombinant Escherichia coli to identify and characterize novel mutations in Goα. Random mutations were generated by polymerase chain reaction in the amino-terminal 56 amino acids of Goα. Guanine nucleotide binding properties of the mutants were assayed in situ and in crude extracts of recombinant E. coli. βy interactions were assayed by pertussis toxin mediated ADP-ribosylation. Efficacy of the screening procedures was evaluated by studying properties of wild-type Goα and site-directed mutations that were characterized previously in other G proteins. Several novel mutants with altered GTP binding characteristics and reduced ability to interact with beta gamma had been isolated from the randomly generated mutant library. ADP-ribosylation of mutants R10G, K21N, and K35E was significantly reduced, whereas two of the mutants bearing multiple amino acid substitutions were refractory to modification. Mutant K35E also exhibited reduced affinity to guanosine 5'-(3-O-thio)triphosphate at submicromolar concentrations of magnesium. These experiments demonstrate the feasibility of using large scale random mutagenesis in the studies of G protein function.