Lipid specificity of the Bacillus thuringiensis toxin/membrane interactions

Abstract

Cyt1A is a cytolytic toxin produced by the spore-forming bacterium Bacillus thuringiensis var. israelensis, used in insecticide preparations. Understanding the molecular details of how the toxin changes conformation in the presence of lipid membrane is important for elucidating the toxin’s mode of action. Previous binding studies were performed using membranes made of chemically undefined lipid egg phosphatidylcholine (PC). Here we studied lipid specificity of Cyt1A binding regarding saturation of fatty acyl chain and chemical nature of the lipid head-group. Fluorescence of tryptophan was used as a measure of binding. We measured Cyt1A binding to membranes made of the saturated lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) at 7°C and 37°C and determined the apparent dissociation constant (Kd) to be 111 ± 21 μM and 19.7 ± 5.9 μM, respectively. The apparent Kd of Cyt1A binding to membranes made of the egg PC lipid mixture was previously determined to be 91 μM. Thus at lower temperature, Cyt1A bin...