Synergism of Bacillus thuringiensis toxins by a fragment of a toxin-binding cadherin.

The insecticidal crystal proteins produced by Bacillus thuringiensis (Bt) are broadly used to control insect pests with agricultural importance. The cadherin Bt-R(1) is a binding protein for Bt Cry1A toxins in midgut epithelia of tobacco hornworm (Manduca sexta). We previously identified the Bt-R(1) region most proximal to the cell membrane (CR12-MPED) as the essential binding region required for Cry1Ab-mediated cytotoxicity. Here, we report that a peptide containing this region expressed in Escherichia coli functions as a synergist of Cry1A toxicity against lepidopteran larvae. Far-UV circular dichroism and (1)H-NMR spectroscopy confirmed that our purified CR12-MPED peptide mainly consisted of beta-strands and random coils with unfolded structure. CR12-MPED peptide bound brush border membrane vesicles with high affinity (K(d) = 32 nM) and insect midgut microvilli but did not alter Cry1Ab or Cry1Ac binding localization in the midgut. By BIAcore analysis we demonstrate that Cry1Ab binds CR12-MPED at high (9 nM)- and low (1 microM)-affinity sites. CR12-MPED-mediated Cry1A toxicity enhancement was significantly reduced when the high-affinity Cry1A-binding epitope ((1416)GVLTLNIQ(1423)) within the peptide was altered. Because the mixtures of low Bt toxin dose and CR12-MPED peptide effectively control target insect pests, our discovery has important implications related to the use of this peptide to enhance insecticidal activity of Bt toxin-based biopesticides and transgenic Bt crops.