Ion channel activity of N-terminal fragments from CryIA(c) delta-endotoxin

Proteolytically-derived amino-terminal fragments from the Bacillus thuringiensis delta-endotoxin, CryIA(c), demonstrated ion channel activity in two in vitro assays in the absence of any membrane receptors. A 24.5 kDa fragment (from Spodoptera frugiperda digestive proteolysis) formed cation-selective channels (10mV for a 3:1 salt gradient) in planar lipid bilayers up to several hundred picoSiemen conductance. A 21.5 kDa fragment doublet (from alkaline hydrolysis) also showed large conductance in planar lipid bilayers and resulted in nearly 2-fold greater 86Rb(+)-K+ efflux from phosphatidylcholine vesicles than 55 kDa CryIA(c). In preliminary screening bioassays, however, the 21.5 kDa fragments showed no insecticidal activity toward neonate Heliothis virescens. Ion channel activity of these putative alpha-helical region fragments support this region being responsible for ion channel properties of the parent delta-endotoxin.