omega-Conotoxin, a calcium channel blocker, inhibits chemotaxis by Escherichia coli. To test whether omega-conotoxin acts at the cytoplasmic membrane, the kinetics of 125I-omega-conotoxin binding was investigated. 125I-omega-Conotoxin bound to Tris-EDTA-permeabilized cells or right-side-out membrane vesicles with saturation kinetics. Binding of 125I-omega-conotoxin to membrane vesicles was inhibited by Ca(2+) ions, but not by Mg(2+) ions. The calA mutant, defective in calcium transport, was more resistant to omega-conotoxin inhibition of chemotaxis than the parental wild-type. 125I-omega-Conotoxin binding to membrane vesicles indicated that both the wild-type and the calA mutant had similar K(D)s for omega-conotoxin binding. However, the saturation level was higher with the calA mutant, indicating that there are more binding sites in the calA mutant. Thus, calA does not directly affect the affinity of the omega-conotoxin binding site. Chemical cross-linking experiments identified two proteins as potential omega-conotoxin receptors.