Wheat tetrameric inhibitors of insect alpha-amylases: Alloploid heterosis at the molecular level

Tetrameric inhibitors of heterologous α-amylases have been characterized in allohexaploid wheat, Triticum aestivum (genomes AABBDD), as well as in Triticum turgidum (AABB) and Triticum tauschii (DD). Their subunits have been identified as the previously described CM proteins. Single oligomeric species were observed in T. Turgidum (subunits CM2, CM3A, and CM16) and in T. tauschii (CM1, CM3D, and CM17) by a two-dimensional electrophoretic method that does not dissociate the inhibitors in the first dimension. Multiple tetrameric species, resulting from different combinations of the subunits contributed by the two ancestral species, are observed by the same procedure in T. aestivum. The three types of subunits were required for significant activity when the inhibitor of T. turgidum was reconstituted from the purified subunits, whereas, in the case of T. tauschii, binary mixtures involving subunit CM1 also had some activity. Additional combinations of the subunits present in these two species, which occur in the allohexaploid T. aestivum, were also reconstituted, and their inhibitory activities ranged from 144% to 33% the activity of the reconstituted inhibitor from T. tauschii. The activity of these inhibitors toward the α-amylase (1,4-α-D-glucan glucanohydrolase, EC 3.2.1.1) of the insect Tenebrio molitor is much greater than that against the salivary enzyme. These observations, together with the previously established chromosomal locations of genes encoding CM proteins, fit a model of alloploid heterosis at the molecular level.