Interaction between functional domains of Bacillus thuringiensis insecticidal crystal proteins

Interactions among the three structural domains of Bacillus thuringiensis C ry1 toxins were investigated by functional analysis of chimeric proteins. H ybrid genes were prepared by exchanging the regions coding for either domai n I or domain III among Cry1Ab, Cry1Ac, Cry1C, and Cry1E. The activity of t he purified trypsin-activated chimeric toxins was evaluated by testing thei r effects on the viability and plasma membrane permeability of Sf9 cells. A mong the parental toxins, only Cry1C was active against these cells and onl y chimeras possessing domain II from Cry1C were functional. Combination of domain I from Cry1E with domains II and III from Cry1C, however, resulted i n an inactive toxin, indicating that domain II from an active toxin is nece ssary, but not sufficient, for activity. Pores formed by chimeric toxins in which domain I was from Cry1Ab or Cry1Ac were slightly smaller than those formed by toxins in which domain I was from Cry1C. The properties of the po res formed by the chimeras are therefore likely to result from an interacti on between domain I and domain II or III. Domain III appears to modulate th e activity of the chimeric toxins: combination of domain III from Cry1Ab wi th domains I and II of Cry1C gave a protein which was more strongly active than Cry1C.