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Original Article

Reductions in Affinity Caused by Knock-down Resistance Mutations

Horia Vais^a, Martin S. Williamson^b, Susannah J. Goodson^b, Alan L. Devonshire^b, Jeffrey W. Warmke^c, Peter N.R. Usherwood^a, and Charles J. Cohen^c

^a From the Division on Molecular Toxicology, School of Biology, University of Nottingham, Nottingham NG7 2RD, United Kingdom
^b Institute of Arable Crops Research-Rothamsted, Harpenden, Herts AL5 2JQ, United Kingdom
^c Merck Research Laboratories, Rahway, New Jersey 07065
Merck Research Laboratories, P.O. Box 2000, Rm. 80N-31C, Rahway, NJ 07065.732-594-3925

cohenc{at}merck.com

kdr and super-kdr are mutations in houseflies and other insects that confer 30- and 500-fold resistance to the pyrethroid deltamethrin. They correspond to single (L1014F) and double (L1014F+M918T) mutations in segment IIS6 and linker II(S4–S5) of Na channels. We expressed Drosophila para Na channels with and without these mutations and characterized their modification by deltamethrin. All wild-type channels can be modified by <10 nM deltamethrin, but high affinity binding requires channel opening: (a) modification is promoted more by trains of brief depolarizations than by a single long depolarization, (b) the voltage dependence of modification parallels that of channel opening, and (c) modification is promoted by toxin II from Anemonia sulcata, which slows inactivation. The mutations reduce channel opening by enhancing closed-state inactivation. In addition, these mutations reduce the affinity for open channels by 20- and 100-fold, respectively. Deltamethrin inhibits channel closing and the mutations reduce the time that channels remain open once drug has bound. The super-kdr mutations effectively reduce the number of deltamethrin binding sites per channel from two to one. Thus, the mutations reduce both the potency and efficacy of insecticide action.

Key Words: insecticide • pyrethroid • para mutation • voltage gated

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