Site-directed mutagenesis and expression in Escherichia coli of WMAI-1, a wheat monomeric inhibitor of insect alpha-amylase.

The wheat monomeric inhibitor WMAI-1 (syn. 0.28) produced in Escherichia coli using the pT7-7 expression vector has the correct N-terminal sequence and the same electrophoretic mobility and specific activity towards the alpha-amylase from the insect Tenebrio molitor as the native WMAI-1 isolated from wheat. This confirms that the native inhibitor is not glycosylated and contradicts claims that a putative glycosyl moiety was essential for inhibition. Thirteen mutants have been obtained at six different sites. Substitution of the highly conserved N-terminal S by the sequence ARIRAR increased the pre-incubation time required for maximum activity. A similar result was obtained by insertion of GPRLPW after position 4, while insertion of EPRAPW at the same position rendered the inhibitor inactive. The substitution D/EGPRL and insertions DGP or D, at position 58, produced complete inactivation. All other mutations had only minor effects on activity.