Analysis of Mutations in the Transmembrane Region of the Aspartate Chemoreceptor in Escherichia coli

Site-specific mutagenesis was used to replace an alanine with a lysine residue and to create a deletion of seven amino acids into the first transmembrane region (TMI region) of the aspartate chemoreceptor in Escherichia coli. The mutations resulted in the loss of aspartate chemotaxis on tryptone motility plates. However, both mutant proteins were able to associate with the membrane and to bind aspartate. They were both refractory to methylation or to modification of the C-terminal region of the protein by the cheB gene product. These results suggested that the integrity of the TMI domain of the protein was required to maintain the function of the cytoplasmic portion of the receptor. The Lys-19 mutant retained the ability to generate a repellent response. Analysis of suppressor mutations of the Lys-19 mutation suggested that formation of an ion pair or specific changes in a 40 amino acid stretch in the cytoplasmic region of the protein (from amino acid 264 to amino acid 303) could suppress the effects of the Lys-19 mutation. The TMI region of the protein may be involved in transmembrane transmission of signals from the periplasmic portion of the cell to the cytoplasmic portion of the Tar protein.