In vivo sulfhydryl modification of the ligand-binding site of Tsr, the Escherichia coli serine chemoreceptor.

The Escherichia coli chemoreceptor Tsr mediates an attractant response to serine. We substituted Cys for Thr-156, one of the residues involved in serine sensing. The mutant receptor Tsr-T156C retained serine- and repellent-sensing abilities. However, it lost serine-sensing ability when it was treated in vivo with sulfhydryl-modifying reagents such as N-ethylmaleimide (NEM). Serine protected Tsr-T156C from these reagents. We showed that [3H]NEM bound to Tsr-T156C and that binding decreased in the presence of serine. By pretreating cells with serine and cold NEM, Tsr-T156C was selectively labeled with radioactive NEM. These results are consistent with the location of Thr-156 in the serine-binding site. Chemical modification of the Tsr ligand-binding site provides a basis for simple purification and should assist further in vivo and in vitro investigations of this chemoreceptor protein.