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Applied and Environmental Microbiology, October 1999, p. 4513-4520, Vol. 65, No. 10
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Identification of Residues in Domain III of
Bacillus thuringiensis Cry1Ac Toxin That Affect Binding
and Toxicity
Mi Kyong
Lee,1
Taek H.
You,1
Fred L.
Gould,2 and
Donald H.
Dean1,*
Department of Biochemistry, The Ohio State
University, Columbus, Ohio 43210,1 and
Department of Entomology, North Carolina State University,
Raleigh, North Carolina 276952
Received 1 April 1999/Accepted 5 August 1999
Alanine substitution mutations in the Cry1Ac domain III region,
from amino acid residues 503 to 525, were constructed to study the
functional role of domain III in the toxicity and receptor binding of
the protein to Lymantria dispar, Manduca sexta,
and Heliothis virescens. Five sets of alanine block mutants
were generated at the residues 503SS504,
506NNI508, 509QNR511,
522ST523, and 524ST525.
Single alanine substitutions were made at the residues
509Q, 510N, 511R, and
513Y. All mutant proteins produced stable toxic fragments
as judged by trypsin digestion, midgut enzyme digestion, and circular
dichroism spectrum analysis. The mutations,
503SS504-AA,
506NNI508-AAA,
522ST523-AA,
524ST525-AA, and 510N-A affected
neither the protein's toxicity nor its binding to brush border
membrane vesicles (BBMV) prepared from these insects. Toward L. dispar and M. sexta, the
509QNR511-AAA, 509Q-A,
511R-A, and 513Y-A mutant toxins showed 4- to
10-fold reductions in binding affinities to BBMV, with 2- to 3-fold
reductions in toxicity. Toward H. virescens, the
509QNR511-AAA, 509Q-A,
511R-A, and 513Y-mutant toxins showed 8- to
22-fold reductions in binding affinities, but only
509QNR511-AAA and 511R-A mutant
toxins reduced toxicity by approximately three to four times. In the
present study, greater loss in binding affinity relative to toxicity
has been observed. These data suggest that the residues
509Q, 511R, and 513Y in domain III
might be only involved in initial binding to the receptor and that the
initial binding step becomes rate limiting only when it is reduced more
than fivefold.
*
Corresponding author. Mailing address: Department of
Biochemistry, The Ohio State University, 484 W. 12th Ave., Columbus, OH
43210. Phone: (614) 292-8829. Fax: (614) 292-3206. E-mail: dean.10{at}osu.edu.
Applied and Environmental Microbiology, October 1999, p. 4513-4520, Vol. 65, No. 10
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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