Random Mutagenesis of G protein ɑ Subunit G_oɑ. Mutations altering nucleotide binding

Abstract

Nucleotide binding properties of the G protein ɑ subunit G_oɑ were probed by mutational analysis in recombinant Escherichia coli. Thousands of random mutations generated by polymerase chain reaction were screened by in situ [^(35)S]GTPyS (guanosine 5'-(3-O-thio)-triphosphate) binding on the colony lifts following transformation of bacteria with modified G_oɑ cDNA. Clones that did not bind the nucleotide under these conditions were characterized by DNA sequence analysis, and the nucleotide binding properties were further studied in crude bacterial extracts. A number of novel mutations reducing the affinity of G_oɑ for GTPyS or Mg^(2+) were identified. Some of the mutations substitute amino acid residues homologous to those known to interact with guanine nucleotides in p21^(ras) proteins. Other mutations show that previously unstudied residues also participate in the nucleotide binding. Several mutants lost GTPyS binding but retained the capacity to interact with the βy subunit complex as determined by pertussis toxin-mediated ADP-ribosylation. One of these, mutant S47C, was functionally expressed in Xenopus laevis oocytes along with the G protein-coupled thyrotropin-releasing hormone (TRH) receptor. Whereas wild-type G_oɑ increased TRH-promoted chloride currents, S47C significantly decreased the hormone-induced Cl^- response, suggesting that this mutation resulted in a dominant negative phenotype