Bacillus thuringiensis Cry4A and Cry4B mosquito-larvicidal proteins: homology-based 3D model and implications for toxin activity.
Three-dimensional (3D) models for the 65-kDa activated Cry4A and Cry4B delta-endotoxins from Bacillus thuringiensis subsp. israelensis that are specifically toxic to mosquito-larvae were constructed by homology modeling, based on atomic coordinates of the Cry1Aa and Cry3Aa crystal structures. They were structurally similar to the known structures, both derived 3D models displayed a three-domain organization: the N-terminal domain (I) is a seven-helix bundle, while the middle and C-terminal domains are primarily comprise of anti-parallel beta-sheets. Circular dichroism spectroscopy confirmed the secondary structural contents of the two homology-based Cry4 structures. A structural analysis of both Cry4 models revealed the following: (a) Residues Arg-235 and Arg-203 are located in the interhelical 5/6 loop within the domain I of Cry4A and Cry4B, respectively. Both are solvent exposed. This suggests that they are susceptible to tryptic cleavage. (b) The unique disulphide bond, together with a proline-rich region within the long loop connecting alpha4 and alpha5 of Cry4A, were identified. This implies their functional significance for membrane insertion. (c) Significant structural differences between both models were found within domain II that may reflect their different activity spectra. Structural insights from this molecular modeling study would therefore increase our understanding of the mechanic aspects of these two closely related mosquito-larvicidal proteins.
Laboratory of Molecular Biophysics, Institute of Molecular Biology and Genetics, Mahidol University, Salaya Campus, Nakornpathom 73170, Thailand. stcas@mahidol.ac.th
Chanan
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Angsuthanasombat
Panapat
P
Uawithya
Somphob
S
Leetachewa
Walairat
W
Pornwiroon
Puey
P
Ounjai
Teerakiat
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Kerdcharoen
Gerd R
GR
Katzenmeier
Sakol
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Panyim
D000330:Aedes / Q000187:drug effects* / Q000502:physiology; D000595:Amino Acid Sequence; D000818:Animals; D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins / Q000737:chemistry / Q000633:toxicity; D001427:Bacterial Toxins / Q000737:chemistry / Q000633:toxicity; D004731:Endotoxins / Q000737:chemistry / Q000633:toxicity; D006460:Hemolysin Proteins; D007814:Larva / Q000187:drug effects*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D015203:Reproducibility of Results; D016415:Sequence Alignment
D000083722:Bacillus thuringiensis Toxins; D001426:Bacterial Proteins; D001427:Bacterial Toxins; D004731:Endotoxins; D006460:Hemolysin Proteins; C030956:insecticidal crystal protein, Bacillus Thuringiensis