Submitted to: Molecular Pharmacology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2004
Publication Date: 2/2/2005
Citation: Tan, J., Liu, Z., Wang, R., Huang, Z., Chen, A.C., Gurevitz, M., Dong, K. 2005. Identification of amino acid residues in the insect sodium channel critical for pyrethroid binding. Molecular Pharmacology. 67(2):513-522. Interpretive Summary: The southern cattle tick (SCT) and the disease it transmits, babesiosis, were eradicated from the US after a 50-year eradication program of dipping cattle and temporarily vacating tick-infested pastures. SCT is still widespread in Mexico and other parts of America. Reintroduction of the tick to the US either unintentionally or by active agro-terrorism could prove devastating to cattle industry. Quarantine by vat dipping with chemical acaricides has prevented reinfestation in the US. But SCT has developed resistance to many pesticides, including pyrethroids. We had shown previously that resistance to pyrethroids was a result of a mutation in the cell membrane protein responsible for sodium transport, but the precise nature of the interaction between the pesticide and the target protein that causes the pesticide to fail was not elucidated. In this report, we show conclusively that this mutation causes the pyrethroid pesticides not to bind the membrane protein. There cannot be any toxic effects when the pesticide does not bind to the target. Better understanding of the interaction between pesticides and their targets will help us design more efficacious pesticides in the future.
Technical Abstract: The voltage-gated sodium channel is the primary target site of pyrethroids, which constitute a major class of insecticides used worldwide. Pyrethroids prolong the opening of sodium channels by inhibiting deactivation and inactivation. Despite numerous attempts to characterize pyrethroid binding to sodium channels in the past several decades, the molecular determinants of the pyrethroid-binding site on the sodium channel remain elusive. Here we showed that an F to I substitution at 1519 (F1519I) in segment 6 of domain III (IIIS6) completely abolished the sensitivity of the cockroach sodium channel expressed in Xenopus oocytes to all eight structurally diverse pyrethroids examined, including permethrin and deltamethrin. In contrast, substitution by tyrosine (Y) or tryptophan (W) reduced the channel sensitivity to deltamethrin only by 3-10 fold, indicating that an aromatic residue at this position is critical for the interaction of pyrethroids with sodium channels. The F1519I mutation, however, did not alter the action of other two classes of sodium channel toxins, batrachotoxin (a site 2 toxin) and Lqhá-IT (a site 3 toxin). Schild analysis utilizing competitive interaction of pyrethroid stereospecific isomers demonstrated that the F1519W mutation and a previously known pyrethroid-resistance mutation, L993F in IIS6, reduced the binding affinity of 1S cis permethrin, an inactive isomer that shares the same binding site with the active isomer 1R cis permethrin. Our results provide the first direct proof that L993 and F1519 are part of the pyrethroid receptor site on an insect sodium channel.