A trimeric building block model for Cry toxins in vitro ion channel formation

The crystal (Cry) insecticidal toxins, or delta-endotoxins, are lethal to a wide variety of insect larvae, and are therefore very important in insect control. Toxicity has been explained by formation of transmembrane oligomeric pores or ion channels and, more recently, by the ability of the monomeric toxin to subvert cellular signaling pathways. The structure, topology, and precise role of the putative pore in toxicity are not known. However, in vitro biophysical studies suggest that helices alpha4 and alpha5 in domain I insert into the lipid bilayer as an alpha-helical hairpin. Mutagenesis studies have assigned an important role to alpha5 in maintaining oligomerization, and to alpha4 in channel formation. To detect the possible homo-oligomerizing tendencies of these two helices, we have used the evolutionary conservation data contained in sixteen Cry homologs in order to filter non-native interactions found during a global conformational search. No conserved homo-oligomer was found for alpha4, but a right handed trimeric alpha5 model was present in the simulations of all Cry sequences. We propose a model for Cry toxin oligomerization based on sequence analysis and available mutagenesis data.