The δ-endotoxin proteins are produced by Bacillus thuringiensis during the sporulation phase of its life cycle. These proteins exhibit insecticidal activity through receptor-mediated ion channel formation. The mode of action of these proteins requires the conversion of the protein from a water-soluble conformation to a membrane-inserted conformation. While there is X-ray structure information for the soluble protein, no detailed structure exists for the membrane-inserted protein. However, based on peptide studies, an umbrella model for the membrane-inserted state has been proposed. Here, we investigated the role of a conserved hydrogen bond interaction between two helices that are suggested to undergo a large conformational change upon membrane insertion. Mutation of either the histidine or the tyrosine resulted in a protein that has significantly reduced bioactivity, increased overall flexibility, and significantly reduced stability. These data highlight an important role for this interaction in the overall stability of the protein. Additionally, the conservation of histidine and tyrosine in these positions may suggest a functional role for the interaction in the conformational switching from soluble to membrane protein. Proteins 2006. © 2005 Wiley-Liss, Inc.