Selection by phage display of a variant mustard trypsin inhibitor toxic against aphids

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Abstract
The mustard trypsin inhibitor, MTI‐2, is a potent inhibitor of trypsin with no activity towards chymotrypsin. MTI‐2 is toxic for lepidopteran insects, but has low activity against aphids. In an attempt to improve the activity of the inhibitor towards aphids, a library of inhibitor variants was constructed and cloned into the pRlac3 phagemid vector. The library of 9.3 × 107 independent colonies was created by randomisation of a stretch of five consecutive codons in the reactive site. Repeated selection rounds against bovine trypsin and chymotrypsin allowed the identification of novel, MTI‐2 derived, antitrypsin and antichymotrypsin inhibitors. Chy8, the selected variant with highest affinity for bovine chymotrypsin (Ki = 32 nm versus >1000 nm for the wild‐type) represents the strongest known recombinant chymotrypsin inhibitor of the MTI‐2 family. It is highly toxic to nymphs of the aphid Acyrthosiphon pisum, and moderately toxic to nymphs of Aphis gossypii and Myzus persicae. The LC50 of 73 µg ml−1 towards A. pisum is the lowest value known among chymotrypsin inhibitors. The aphicidal activity of Chy8 was improved eightfold compared to the wild‐type inhibitor. This demonstrates, for the first time, that bovine chymotrypsin provides a useful template to select engineered proteins highly toxic against these aphids. The selected gene will allow the development of transgenic crops that are protected against sucking insect pests.