RESUMO
The delta-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.
Assuntos
Bacillus thuringiensis/química , Bacillus thuringiensis/metabolismo , Endotoxinas/química , Endotoxinas/metabolismo , Canais Iônicos/química , Canais Iônicos/metabolismo , Sequência de Aminoácidos , Bacillus thuringiensis/genética , Sequência Conservada , Endotoxinas/genética , Histidina/genética , Histidina/metabolismo , Canais Iônicos/genética , Dados de Sequência Molecular , Mutação/genética , Estrutura Secundária de Proteína , Alinhamento de Sequência , Tirosina/genética , Tirosina/metabolismoRESUMO
For almost half a century, the structure of the full-length Bacillus thuringiensis (Bt) insecticidal protein Cry1Ac has eluded researchers, since Bt-derived crystals were first characterized in 1965. Having finally solved this structure we report intriguing details of the lattice-based interactions between the toxic core of the protein and the protoxin domains. The structure provides concrete evidence for the function of the protoxin as an enhancer of native crystal packing and stability.