Structure and function of an insect alpha-carboxylesterase (alpha Esterase7) associated with insecticide resistance

Abstract

Insect carboxylesterases from the alpha Esterase gene cluster, such as alpha E7 (also known as E3) from the Australian sheep blowfly Lucilia cuprina (Lc alpha E7), play an important physiological role in lipid metabolism and are implicated in the detoxification of organophosphate (OP) insecticides. Despite the importance of OPs to agriculture and the spread of insect-borne diseases, the molecular basis for the ability of alpha-carboxylesterases to confer OP resistance to insects is poorly understood. In this work, we used laboratory evolution to increase the thermal stability of Lc alpha E7, allowing its overexpression in Escherichia coli and structure determination. The crystal structure reveals a canonical alpha/beta-hydrolase fold that is very similar to the primary target of OPs (acetylcholinesterase) and a unique N-terminal alpha-helix that serves as a membrane anchor. Soaking of Lc alpha E7 crystals in OPs led to the capture of a crystallographic snapshot of Lc alpha E7 in its phosphorylated state, which allowed comparison with acetylcholinesterase and rationalization of its ability to protect insects against the effects of OPs. Finally, inspection of the active site of Lc alpha E7 reveals an asymmetric and hydrophobic substrate binding cavity that is well-suited to fatty acid methyl esters, which are hydrolyzed by the enzyme with specificity constants (similar to 10(6) M-1 s(-1)) indicative of a natural substrate.